isl_tab_pip.c: find_div: reserve negative return value for failures
isl_tab_pip.c: find_div: reserve negative return value for failures

In particular, if no integer division can be found, then
return a position beyond the integer divisions instead of a negative value.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl