stagefright: Remove assert for corrupt clips for AMRNB

Corrupt AMR NB clips were crashing media player
This was due to the assert in the AMRNB decoder.
To avoid this, post error and exit if corrupt
clip is played.
CRs-Fixed: 486681

Change-Id: I8f6809a896b91aaffa8f5ae17d8ae4a3920a4b73