IMediaExtractor: ensure users to check returned value by getTrack. Test: fix the crash from the stream in the bug. Bug: 37777357 Bug: 23270724 Change-Id: Ia99907c396a2074a9419db9b8103a9325b290169 (cherry picked from commit 5472aee4d2375432e2dde05ab76dd47fa3bac536)