Roll ffmpeg DEPs for security fixes. Add regression tests.

Roll ffmpeg DEPs for security fixes. Add regression tests.

Pulls in the following fixes:
b9d631d avformat/utils: Clear pointer in ff_alloc_extradata() to avoid leaving a stale pointer in memory
7575fa1 avfilter/buffer: use av_freep() to avoid leaving stale pointers in memory
0aa6981 avformat/matroskadec: Use av_freep() to avoid leaving stale pointers in memory
e860c4c avformat/mov: use av_freep() to avoid leaving stale pointers in memory
c61fcd2 avformat/mov: Clear array to prevent potential out of array read from av_dlog()
a6dd29c avformat/mov: fix integer overflow in mov_read_udta_string()
878627f mov: Fix negative size calculation in mov_read_default().
c179d68 mov: Fix overflow and error handling in read_tfra().

BUG=444533,444522,444539,444546
TEST=regression tests, test cases no longer crash
TBR=scherkus

Review URL: https://codereview.chromium.org/838653002

Cr-Commit-Position: refs/heads/master@{#310173}