SampleIterator: fix sizeof bug in getSampleSizeDirect.

Incorrect sizeof(*size) in SampleIterator::getSampleSizeDirect
potentially leads to file size overflow.

Test: Create m4a file with stsz box in the end of file and play it.

Change-Id: Ic8a073de2dfc35890ce65a03dd5ed9343a6ff007

Signed-off-by: Jia Jia <jia.jia@zte.com.cn>