MPEG4Writer: Fix unsigned integer overflow

When the timestampUs is negative and firstSampleStartOffsetUs is 0
then we convert it into a positive number which causes an overflow
if the timestamp is equal to INT_MIN.

Test: ./ndk_mediamuxer_fuzzer

Bug: 336685906
Bug: 346056654

Change-Id: I93060a8c3ec31e4309d3b49e528eeda8c7fc88d2