Prevent illegal positions for tmp_at

It seems like m_throw() can call tmp_at() with !isok coordinates,
so prevent those in both functions

src/mthrowu.c

@@ -440,7 +440,7 @@ struct obj *obj;		/* missile (or stack providing it) */
 		tmp_at(bhitpos.x, bhitpos.y);
 		delay_output();
 	}
-	tmp_at(bhitpos.x, bhitpos.y);
+	if (isok(bhitpos.x, bhitpos.y)) tmp_at(bhitpos.x, bhitpos.y);
 	delay_output();
 	tmp_at(DISP_END, 0);