Fix potential size overflow in blockimg.cpp

Switch to 64 bit integers since the size of the entire src/tgt images may
not fit in size_t of ILP32.

There are other theoretical overflow cases in memory allocation and I/O
functions. However, they reside within a single transfer command and are
less likely to happen. I will evaluate and address them in separate
cls.

Test: unit tests pass
Bug: 122461124
Change-Id: Ib719ee695920877458fcfaa25c6ac058a5bbabf2