flac parser asserts on flac file with large hole of zero bytes

[Mattias Wadman]

Summary of the bug:

Corrupt flac files with large hole of zero bytes causes libavcodec flac parser to assert.

How to reproduce:

# create a 540MB test file with a large (> 0x20000000) zero bytes hole in the middle
$ ffmpeg -y -f lavfi -i anoisesrc -t 4000 test.flac
$ dd if=/dev/zero conv=notrunc of=test.flac oseek=20000 bs=1024 count=540000

# try decode it
$ ffmpeg -i test.flac -f null -
...
Assertion index > -0x20000000 failed at libavcodec/parser.c:165

Reproducible with 5.1.1 and master (b881d2db8892e88a625dfa4ac2d5b8ca53ab9595)

The large negative error code seems to come here ​https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/flac_parser.c#L653. Based on the assert and comment in libavcodec/parser.c i get a feeling there should be some limit on position delta or pos should be signalled in some other way?

[Elon Musk]

Fixed in cf2cf31805448dd11692313440a21821773a6128