Exit loop ealier when matching lastoffset.

|lastoffset| and |oldsize| doesn't change in this loop, no point keep
incrementing |scsc| and comparing.

bsdiff is about 2% faster with no change on the patch content.

Bug: 77817425
Test: bsdiff_unittest
Change-Id: Ie6a5ddc4b8805ed76c9c626fa75010617b7975ef