When heif_context* doesn't read from a file, `HeifFile::m_input_stream` never initialized. Therefore, calling `heif_decode_image()` results in an unexpected Segmentation Fault.

```C
  // Read HEIF
  heif_context* ctx_read = heif_context_alloc();
  heif_context_read_from_file(ctx_read, filename, nullptr);

  heif_image_handle* handle;
  heif_context_get_primary_image_handle(ctx_read, &handle);

  heif_image* img;
  heif_decode_image(handle, &img, heif_colorspace_RGB, heif_chroma_interleaved_RGB, nullptr);

  // Write HEIF
  heif_encoder* encoder;
  heif_context* ctx_write = heif_context_alloc();
  heif_context_get_encoder_for_format(ctx_write, heif_compression_HEVC, &encoder);

  // 1. Encode image
  heif_context_encode_image(ctx_write, img, encoder, nullptr, &handle);

  // 2. Get image back
  heif_decode_image(handle, &img, heif_colorspace_RGB, heif_chroma_interleaved_RGB, nullptr); // SEGFAULT
```

I would like this to work, or at least have `struct heif_error` catch it.
Encode then Decode Crash
This is a GitHub Issue
repo:libheif
owner:strukturag
Title : Encode then Decode Crash
Issue date:

--- start body ---
When heif_context* doesn't read from a file, `HeifFile::m_input_stream` never initialized. Therefore, calling `heif_decode_image()` results in an unexpected Segmentation Fault.

```C
  // Read HEIF
  heif_context* ctx_read = heif_context_alloc();
  heif_context_read_from_file(ctx_read, filename, nullptr);

  heif_image_handle* handle;
  heif_context_get_primary_image_handle(ctx_read, &handle);

  heif_image* img;
  heif_decode_image(handle, &img, heif_colorspace_RGB, heif_chroma_interleaved_RGB, nullptr);

  // Write HEIF
  heif_encoder* encoder;
  heif_context* ctx_write = heif_context_alloc();
  heif_context_get_encoder_for_format(ctx_write, heif_compression_HEVC, &encoder);

  // 1. Encode image
  heif_context_encode_image(ctx_write, img, encoder, nullptr, &handle);

  // 2. Get image back
  heif_decode_image(handle, &img, heif_colorspace_RGB, heif_chroma_interleaved_RGB, nullptr); // SEGFAULT
```

I would like this to work, or at least have `struct heif_error` catch it.
--- end body ---
