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
Bug: 142641801
Test: MTBF test and make sure
      no assertion in AMediaFormat_delete(mMeta) of ~AACExtractor()
Change-Id: Ie04989f20170391961c8a6d12e1882262be18d7f
Merged-In: Ie04989f20170391961c8a6d12e1882262be18d7f
(cherry picked from commit 19e92b5b)