This issue looks similar to Bug ID -- 759336. So, merging the issue into same.
Please un-merge if not the same.
Thank You.

ClusterFuzz has detected this issue as fixed in range 500054:500109.

Detailed report: https://clusterfuzz.com/testcase?key=5713298032164864

Fuzzer: libFuzzer_mediasource_WEBM_VP9_pipeline_integration_fuzzer
Job Type: libfuzzer_chrome_ubsan
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  buffers_.empty() || CanAppendBuffersToEnd(new_buffers, new_buffers_group_start_t
  media::SourceBufferRange::AppendBuffersToEnd
  media::SourceBufferStream::Append
  
Sanitizer: undefined (UBSAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=497087:497155
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_ubsan&range=500054:500109

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5713298032164864

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

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