This was found while testing with m-c:
BuildID=20190318220034
SourceStamp=2abb636ad481768b7c88619080cf224b2c266b2d

==114823== Conditional jump or move depends on uninitialised value(s)
==114823==    at 0x140FCAE4: mp4parse::read_sinf (ptr.rs:204)
==114823==    by 0x140F4150: mp4parse::read_stsd (lib.rs:1988)
==114823==    by 0x140EDA29: mp4parse::read_mdia (lib.rs:976)
==114823==    by 0x140E7377: mp4parse::read_moov (lib.rs:870)
==114823==    by 0x140E4C11: mp4parse_read (lib.rs:729)
==114823==    by 0x11D3D5A3: mozilla::MP4Metadata::Parse() (MP4Metadata.cpp:102)
==114823==    by 0x11D3C2F0: mozilla::MP4Demuxer::Init() (MP4Demuxer.cpp:145)
==114823==    by 0x11B019E3: mozilla::detail::ProxyFunctionRunnable<mozilla::MediaFormatReader::DemuxerProxy::Init()::$_15, mozilla::MozPromise<mozilla::MediaResult, mozilla::MediaResult, true> >::Run() (MediaFormatReader.cpp:898)
==114823==    by 0xFD42D43: mozilla::TaskQueue::Runner::Run() (TaskQueue.cpp:199)
==114823==    by 0xFD533B3: nsThreadPool::Run() (nsThreadPool.cpp:241)
==114823==    by 0xFD534FC: non-virtual thunk to nsThreadPool::Run() (nsThreadPool.cpp:0)
==114823==    by 0xFD4FCD7: nsThread::ProcessNextEvent(bool, bool*) (nsThread.cpp:1179)
==114823==  Uninitialised value was created by a stack allocation
==114823==    at 0x140FB9BA: mp4parse::read_sinf (lib.rs:2198)
==114823==

This looks similar to Bug 1538580.

This is likely a false positive that was recently fixed in Valgrind. I can confirm this is no longer reproducible with the latest version of Valgrind.

I tested with m-c 20200102-c7082b580eeb and Valgrind commit 2a7d3ae7681.