MPEG4Extractor.cpp: handle chunk_size > SIZE_MAX

chunk_size is a uint64_t, so it can legitimately be bigger
than SIZE_MAX, which would cause the subtraction to underflow.

https://code.google.com/p/android/issues/detail?id=182251

Bug: 23034759
Change-Id: Ic1637fb26bf6edb0feb1bcf2876fd370db1ed547
