MMapTrack: Fix nullptr crash when ATRACE audio is enabled

MMapTrack and PatchTracks may have null clients.

Flag: EXEMPT bugfix
Test: Perfetto with Oboetester
Bug: 383354934
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a6a2410be108d99db3875a3f9cedf60481772487)
Merged-In: Ia66329d60293f538aed4fa480f97d88404204d2b
Change-Id: Ia66329d60293f538aed4fa480f97d88404204d2b
1 file changed