Fix out-of-range if first sample is not sync

[bsantos]

mp4track output of first 3 samples of offending file without fix:

operator[]: illegal array index: 2147483647 of 4: errno: 34 (../mp4v2/src/mp4array.h,130)
sampleId 1, size 27 duration 2939 time 0 00:00:00.000
sampleId 2, size 24361 duration 5041 time 2939 00:00:00.032 S
sampleId 3, size 19542 duration 2880 time 7980 00:00:00.088

mp4track output of first 3 samples of offending file with fix:

sampleId 1, size 27 duration 2939 time 0 00:00:00.000
sampleId 2, size 24361 duration 5041 time 2939 00:00:00.032 S
sampleId 3, size 19542 duration 2880 time 7980 00:00:00.088

The video in question was created by a Sony Xperia L1 model g3311.

Note: I can't provide the file because of privacy reasons (it came from a customer).