libavformat/matroskaenc.c:688: error: Null Dereference
  pointer `mkv->md5_ctx` last assigned on line 687 could be null and is dereferenced by call to `av_md5_init()` at line 688, column 5.

libavformat/matroskaenc.c:679:1: start of procedure mkv_write_header()
677. }
678. 
679. static int mkv_write_header(AVFormatContext *s)
     ^
680. {
681.     MatroskaMuxContext *mkv = s->priv_data;

libavformat/matroskaenc.c:681:5: 
679. static int mkv_write_header(AVFormatContext *s)
680. {
681.     MatroskaMuxContext *mkv = s->priv_data;
         ^
682.     ByteIOContext *pb = s->pb;
683.     ebml_master ebml_header, segment_info;

libavformat/matroskaenc.c:682:5: 
680. {
681.     MatroskaMuxContext *mkv = s->priv_data;
682.     ByteIOContext *pb = s->pb;
         ^
683.     ebml_master ebml_header, segment_info;
684.     AVMetadataTag *tag;

libavformat/matroskaenc.c:687:5: 
685.     int ret;
686. 
687.     mkv->md5_ctx = av_mallocz(av_md5_size);
         ^
688.     av_md5_init(mkv->md5_ctx);
689.     mkv->tracks = av_mallocz(s->nb_streams * sizeof(*mkv->tracks));

libavutil/mem.c:157:1: start of procedure av_mallocz()
  155. }
  156. 
  157. void *av_mallocz(unsigned int size)
       ^
  158. {
  159.     void *ptr = av_malloc(size);

libavutil/mem.c:159:5: 
  157. void *av_mallocz(unsigned int size)
  158. {
  159.     void *ptr = av_malloc(size);
           ^
  160.     if (ptr)
  161.         memset(ptr, 0, size);

libavutil/mem.c:64:1: start of procedure av_malloc()
    62.    linker will do it automatically. */
    63. 
    64. void *av_malloc(unsigned int size)
        ^
    65. {
    66.     void *ptr = NULL;

libavutil/mem.c:66:5: 
    64. void *av_malloc(unsigned int size)
    65. {
    66.     void *ptr = NULL;
            ^
    67. #if CONFIG_MEMALIGN_HACK
    68.     long diff;

libavutil/mem.c:72:8: Taking true branch
    70. 
    71.     /* let's disallow possible ambiguous cases */
    72.     if(size > (INT_MAX-16) )
               ^
    73.         return NULL;
    74. 

libavutil/mem.c:73:9: 
    71.     /* let's disallow possible ambiguous cases */
    72.     if(size > (INT_MAX-16) )
    73.         return NULL;
                ^
    74. 
    75. #if CONFIG_MEMALIGN_HACK

libavutil/mem.c:117:1: return from a call to av_malloc
    115. #endif
    116.     return ptr;
    117. }
         ^
    118. 
    119. void *av_realloc(void *ptr, unsigned int size)

libavutil/mem.c:160:9: Taking false branch
  158. {
  159.     void *ptr = av_malloc(size);
  160.     if (ptr)
               ^
  161.         memset(ptr, 0, size);
  162.     return ptr;

libavutil/mem.c:162:5: 
  160.     if (ptr)
  161.         memset(ptr, 0, size);
  162.     return ptr;
           ^
  163. }
  164. 

libavutil/mem.c:163:1: return from a call to av_mallocz
  161.         memset(ptr, 0, size);
  162.     return ptr;
  163. }
       ^
  164. 
  165. char *av_strdup(const char *s)

libavformat/matroskaenc.c:688:5: 
686. 
687.     mkv->md5_ctx = av_mallocz(av_md5_size);
688.     av_md5_init(mkv->md5_ctx);
         ^
689.     mkv->tracks = av_mallocz(s->nb_streams * sizeof(*mkv->tracks));
690. 

libavutil/md5.c:118:1: start of procedure av_md5_init()
  116. }
  117. 
  118. void av_md5_init(AVMD5 *ctx){
       ^
  119.     ctx->len    = 0;
  120. 

libavutil/md5.c:119:5: 
  117. 
  118. void av_md5_init(AVMD5 *ctx){
  119.     ctx->len    = 0;
           ^
  120. 
  121.     ctx->ABCD[0] = 0x10325476;
