APNG decoder incorrectly assumes fcTL chunk must be immediately followed by fdAT or IDAT

[tobbez]

The APNG decoder (more specifically apng_read_packet in libavformat/apngdec.c) incorrectly assumes an fcTL chunk must be immediately followed by either an fdAT or an IDAT chunk.

In addition, when the fdAT/IDAT check fails, the input stream is not rewinded to the start of the chunk that follows the fcTL chunk, causing misleading log messages: "tag=ware (0x65726177) len=1399809652" in the output below ought to be "tag=tEXt (0x74584574) len=16".

An example file can be created using exiftool on an existing APNG file (exiftool -Software=example apng.png). This produces a file where the first fcTL chunk is followed by a tEXt chunk. Such an example will be attached.

How to reproduce:

% ffmpeg -i example.png -f null /dev/null
ffmpeg version N-115183-g02c032abcd Copyright (c) 2000-2024 the FFmpeg developers
  built with gcc 13 (13.2.1_p20240503 p15)
  configuration:
  libavutil      59. 17.100 / 59. 17.100
  libavcodec     61.  5.103 / 61.  5.103
  libavformat    61.  3.103 / 61.  3.103
  libavdevice    61.  2.100 / 61.  2.100
  libavfilter    10.  2.101 / 10.  2.101
  libswscale      8.  2.100 /  8.  2.100
  libswresample   5.  2.100 /  5.  2.100
[apng @ 0x55841a01a7c0] Could not find codec parameters for stream 0 (Video: apng, none, 64x64): unspecified pixel format
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, apng, from 'example.png':
  Duration: N/A, bitrate: N/A
  Stream #0:0: Video: apng, none, 64x64, 100k tbr, 100k tbn
Stream mapping:
  Stream #0:0 -> #0:0 (apng (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
[apng @ 0x55841a01a7c0] In-stream tag=ware (0x65726177) len=1399809652 is not implemented. Update your FFmpeg version to the newest one from Git. If the problem still occurs, it means that your file has a feature which has not been implemented.
[apng @ 0x55841a01a7c0] If you want to help, upload a sample of this file to https://streams.videolan.org/upload/ and contact the ffmpeg-devel mailing list. (ffmpeg-devel@ffmpeg.org)
[in#0/apng @ 0x55841a01a500] Error during demuxing: Not yet implemented in FFmpeg, patches welcome
Cannot determine format of input 0:0 after EOF
[vf#0:0 @ 0x55841a01cb80] Task finished with error code: -1094995529 (Invalid data found when processing input)
[vf#0:0 @ 0x55841a01cb80] Terminating thread with return code -1094995529 (Invalid data found when processing input)
[vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Could not open encoder before EOF
[vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Task finished with error code: -22 (Invalid argument)
[vost#0:0/wrapped_avframe @ 0x55841a01cfc0] Terminating thread with return code -22 (Invalid argument)
[out#0/null @ 0x55841a01c940] Nothing was written into output file, because at least one of its streams received no packets.
frame=    0 fps=0.0 q=0.0 Lsize=       0KiB time=N/A bitrate=N/A speed=N/A
Conversion failed!