Predator and CL could not provide any possible suspects.
Using the code search for the file, “mpegaudiodec_template.c” assigning to concern owner from GIT blame.
Suspecting Commit#
https://chromium.googlesource.com/chromium/third_party/ffmpeg.git/+/8ebed703f153e979edb2156754c8bdac4d5d6266

@wolenetz -- Could you please look into this issue as. Assigning this issue to you as per the issue 639840 and we were unable to assign this issue to michael@ (CC'd).
Thank You.

ClusterFuzz has detected this issue as fixed in range 515364:515426.

Detailed report: https://clusterfuzz.com/testcase?key=5070575054880768

Fuzzer: libFuzzer_mediasource_MP3_pipeline_integration_fuzzer
Job Type: libfuzzer_chrome_ubsan
Platform Id: linux

Crash Type: Undefined-shift
Crash Address: 
Crash State:
  l3_unscale
  huffman_decode
  mp_decode_layer3
  
Sanitizer: undefined (UBSAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=513674:513694
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=515364:515426

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5070575054880768

See https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reference.md for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.

ClusterFuzz testcase 5070575054880768 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.

It looks like the M64 ffmpeg roll fixed this issue, or at least changed the repro stack.