An issue was discovered in Bento4 through 1.6.0-639. A NULL pointer dereference occurs in AP4_StszAtom::WriteFields.
Hi there, I use my fuzzer for fuzzing the binary mp4decrypt, and this binary crashes with the following:
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==24087==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000702ee8 bp 0x7ffcf40a75f0 sp 0x7ffcf40a73b0 T0)
==24087==The signal is caused by a READ memory access.
==24087==Hint: address points to the zero page.
    #0 0x702ee8 in AP4_StszAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x702ee8)
    #1 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #2 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #3 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #4 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #5 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #6 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #7 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #8 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #9 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #10 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #11 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #12 0x62cea7 in AP4_Processor::Process(AP4_ByteStream&, AP4_ByteStream&, AP4_ByteStream*, AP4_Processor::ProgressListener*, AP4_AtomFactory&) (/fuzztest/mp4decrypt/mp4decrypt+0x62cea7)
    #13 0x412846 in main (/fuzztest/mp4decrypt/mp4decrypt+0x412846)
    #14 0x7fcaa49f1c86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21c86)
    #15 0x407c99 in _start (/fuzztest/mp4decrypt/mp4decrypt+0x407c99)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/fuzztest/mp4decrypt/mp4decrypt+0x702ee8) in AP4_StszAtom::WriteFields(AP4_ByteStream&)
==24087==ABORTING
```

### System Details
Test Machine: Ubuntu 18.04 (docker)
Project Name: mp4decrypt (Bento4-1.6.0-639)

### Command
./mp4decrypt mp4decrypt.demo /dev/null

### Poc
[mp4decrypt_Poc.zip](https://github.com/axiomatic-systems/Bento4/files/9591036/mp4decrypt_Poc.zip)


### Credit
Wanying Cao(NCNIPC of China), (Zhongguancun Laboratory)
Han Zheng (NCNIPC of China, [Hexhive](http://hexhive.epfl.ch/)), (Zhongguancun Laboratory)
SEGV error
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : SEGV error
Issue date:

--- start body ---
Hi there, I use my fuzzer for fuzzing the binary mp4decrypt, and this binary crashes with the following:
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==24087==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000702ee8 bp 0x7ffcf40a75f0 sp 0x7ffcf40a73b0 T0)
==24087==The signal is caused by a READ memory access.
==24087==Hint: address points to the zero page.
    #0 0x702ee8 in AP4_StszAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x702ee8)
    #1 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #2 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #3 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #4 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #5 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #6 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #7 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #8 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #9 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #10 0x4fc423 in AP4_ContainerAtom::WriteFields(AP4_ByteStream&) (/fuzztest/mp4decrypt/mp4decrypt+0x4fc423)
    #11 0x82facf in AP4_AtomListWriter::Action(AP4_Atom*) const (/fuzztest/mp4decrypt/mp4decrypt+0x82facf)
    #12 0x62cea7 in AP4_Processor::Process(AP4_ByteStream&, AP4_ByteStream&, AP4_ByteStream*, AP4_Processor::ProgressListener*, AP4_AtomFactory&) (/fuzztest/mp4decrypt/mp4decrypt+0x62cea7)
    #13 0x412846 in main (/fuzztest/mp4decrypt/mp4decrypt+0x412846)
    #14 0x7fcaa49f1c86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21c86)
    #15 0x407c99 in _start (/fuzztest/mp4decrypt/mp4decrypt+0x407c99)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/fuzztest/mp4decrypt/mp4decrypt+0x702ee8) in AP4_StszAtom::WriteFields(AP4_ByteStream&)
==24087==ABORTING
```

### System Details
Test Machine: Ubuntu 18.04 (docker)
Project Name: mp4decrypt (Bento4-1.6.0-639)

### Command
./mp4decrypt mp4decrypt.demo /dev/null

### Poc
[mp4decrypt_Poc.zip](https://github.com/axiomatic-systems/Bento4/files/9591036/mp4decrypt_Poc.zip)


### Credit
Wanying Cao(NCNIPC of China), (Zhongguancun Laboratory)
Han Zheng (NCNIPC of China, [Hexhive](http://hexhive.epfl.ch/)), (Zhongguancun Laboratory)
--- end body ---
