Fixes #5613 null acid loc

Fixes #5613

Loc checking has been replaced with coord-alignments of the target; this fixes acid being incorrectly placed on walls since their loc is an area.

The 150-200 spawn that was stopping the acid's position from updating in time with a target being removed has been replaced with target_strength values being multiplied by 80. The time it takes for acid to melt something is a bit shorter and less random.
&& src.loc