Remember position of monster you hit with polearm

If you hit a monster with a polearm, following polearm applications
will try to automatically suggest the monster's position

src/do.c

@@ -1402,6 +1402,7 @@ boolean at_stairs, falling, portal;
 	/* assume this will always return TRUE when changing level */
 	(void) in_out_region(u.ux, u.uy);
 	(void) pickup(1);
+	context.polearm.hitmon = NULL;
 }
 
 STATIC_OVL void