MPEG4Extractor: ensure returned status is checked.

Also fix handling of zero atom size in MPEG4Source::parseChunk.
IDataSource: ensure readAt returns correct status.
Test: manually test with mediaplayer.
Bug: 34718515
Change-Id: I1219ec579aa0876dc1230e36af46b158b84c6d77

(cherry picked from commit ff1fb4d5)
(cherry picked from commit 37156121)

Change-Id: I51546975ac0992cff7cf890a71a177e1058ed613
CVE-2017-0774