Delete an uninitialized object in AACExtractor

Due to some reason, for example bad file, readAt header fails
in AACExtractor(...) and the variable mMeta is not initialized.
Then exception happens when to delete mMeta in ~AMRExtractor().

Bug: 142630125
Test: MTBF test and make sure
      no assertion in AMediaFormat_delete(mMeta) of ~AACExtractor()
Change-Id: Ie04989f20170391961c8a6d12e1882262be18d7f