Fix multiple division-by-zero conditions in MPEG4 parsing

Several situations arise processing MP4 atoms that lead to undefined behavior
when dividing by zero. Typically this results in a crash (denial of service
condition).

NOTE: In most cases we simply avoid the division, leaving kKeyDuration unset.
It may be more desirable to bail out, as we do in the parseSegmentIndex case.

Bug: 20139950
Change-Id: I62e1b977f0e5ed0094094a55d300bac76b476c7b