Fix integer overflow when handling MPEG4 tx3g atom

When the sum of the 'size' and 'chunk_size' variables is larger than 2^32,
an integer overflow occurs. Using the result value to allocate memory
leads to an undersized buffer allocation and later a potentially
exploitable heap corruption condition. Ensure that integer overflow does
not occur.

CYNGNOS-446
Change-Id: Id050a36b33196864bdd98b5ea24241f95a0b5d1f