Avoid crash for stss sync sample number 0

A sample number value of 0 means that the value stored in
the mSyncSamples array, would become negative (-1),
when converted to index value. This causes a crash.

Make sure that stss sample numbers are bigger
than 0 before converting sample number to index value.

Bug: 32423862
bug: 35645051
Test: Playback video that triggers stss sync sample number 0
Change-Id: I35bee7c718e01b086d7e05deda13b38083f509f5