Check monster relocation degenerate cases more strictly

Just in case we're trying to put the monster on the same location
where it already thinks it is... but actually isn't.

src/mon.c

@@ -2586,7 +2586,7 @@ boolean move_other; /* make sure mtmp gets to x, y! so move m_at(x, y) */
     xchar newx, newy;
     coord mm;
 
-    if (mtmp->mx == x && mtmp->my == y)
+    if (mtmp->mx == x && mtmp->my == y && m_at(x,y) == mtmp)
         return TRUE;
 
     if (move_other && (othermon = m_at(x, y)) != 0) {

src/teleport.c

@@ -1005,8 +1005,8 @@ register int x, y;
     register int oldx = mtmp->mx, oldy = mtmp->my;
     boolean resident_shk = mtmp->isshk && inhishop(mtmp);
 
-    if (x == mtmp->mx && y == mtmp->my) /* that was easy */
-        return;
+    if (x == mtmp->mx && y == mtmp->my && m_at(x,y) == mtmp)
+        return; /* that was easy */
 
     if (oldx) { /* "pick up" monster */
         if (mtmp->wormno) {