mp4: fix potential dereferences of NULL

clang's static analyzer wasn't happy about the potential load of NULL
from mLastTrack on line (after this patch) 4606:

> frameworks/av/media/extractors/mp4/MPEG4Extractor.cpp:4663:32:
warning: Access to field 'meta' results in a dereference of a null
pointer (loaded from field 'mLastTrack')
[clang-analyzer-core.NullDereference]

Since there's a NULL check above, the analyzer assumes `mLastTrack` may be
NULL here. This doesn't seem like an unreasonable assumption to me.

While I'm in the area, looks like we may have the same issue with MP3,
so I've opportunistically added a NULL check there.

Bug: None
Test: TreeHugger
Change-Id: I1e1be8bdb36fd059e1f4eabc165650389d02e45e