libstagefright: Fix to handle corrupted .m4a clips

- Some m4a clips with corrupted header causes native crash
  in mediaserver.
- This is due to a wrong sample size which had to read from the clip.
- Fixed this by checking for the sample size extracted from the clip
  against the mediabuffer size.

CRs-fixed: 298469
(cherry picked from commit fc3c44a20a78449a2746f30a10e60668954fbb79)

Change-Id: I17418307d3de98c123fef0366c76db6bee5526d0