webp demux failure

[jzern]

Summary of the bug:

If a valid webp file contains extra data after the reported file (riff) size the demuxer will fail, see below. This can occur in some transcode pipeline if e.g., padding bytes from encryption are not stripped. libwebp tools handle the attached as they only parse through the reported riff size.

How to reproduce:

% ./ffmpeg -y -i test.webp out.ppm
ffmpeg version N-94694-g95780f4dcb Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 8 (Debian 8.2.0-14+build1)
configuration: --enable-libwebp
libavutil      56. 33.100 / 56. 33.100
libavcodec     58. 56.100 / 58. 56.100
libavformat    58. 31.104 / 58. 31.104
libavdevice    58.  9.100 / 58.  9.100
libavfilter     7. 58.101 /  7. 58.101
libswscale      5.  6.100 /  5.  6.100
libswresample   3.  6.100 /  3.  6.100
Input #0, webp_pipe, from 'test.webp':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: webp, yuv420p(tv, bt470bg/unknown/unknown), 128x128, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (webp (native) -> ppm (native))
Press [q] to stop, [?] for help
[webp @ 0x55c25b430980] Multiple ff_thread_finish_setup() calls
[webp @ 0x55c25b430980] A frame threaded decoder did not free the frame on failure. This is a bug, please report it.
Assertion frame->buf[0] failed at src/libavcodec/decode.c:618

[Carl Eugen Hoyos]

Duplicate of ticket #7612.