More monster location sanity checks

Placing a monster over already existing one will now complain.
Define EXTRA_SANITY_CHECKS to check monster removal too.

src/mhitm.c

@@ -725,6 +725,7 @@ register struct attack *mattk;
      *  but don't leave it on the screen.  Move the aggressor to the
      *  defender's position.
      */
+    remove_monster(dx, dy);
     remove_monster(ax, ay);
     place_monster(magr, dx, dy);
     newsym(ax, ay); /* erase old position */