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.