stagefright: Stability fixes

Add a null check around MediaCodec release to avoid the warning
from AMessage about target handler being gone.

Add bufferlock in updateBuffer to avoid race condition between
info->mFormat being assigned to mOutputFormat while mOutputFormat
is being updated at the same time.

Change-Id: Ibc5a9a79a840d1c15a99be7c74ded1588edf3918