Fix a possible divide-by-zero.

We were attempting to ensure a minimum spacing between granule positions when guessing the start of a link location. However, we took a strictly-positive granule position, added a fixed increment with op_granpos_add(), and checked if op_granpos_add() failed. op_granpos_add() only fails if the sum would have overflowed past zero, which can never happen when adding two strictly positive granule positions. Instead, we need to check if the result becomes negative (which is a legal granule position, but violates our assumptions in the search).

Thanks to Felicia Lim for the report.