Fix null-pointer-dereferences accessing the SampleTable

While processing various sample table related FourCC values, methods are called
on a NULL mLastTrack or sampleTable object. This leads to undefined behavior
which typically results in a crash (denial of service condition).

Bug: 20139950
Change-Id: Ie2dd8222e702d8bf95faf7d2bd44e6303cd21f68
1 file changed