Fix migrating monster throwing a potion

When a monster did something trying to get out of a boulder fort,
it usually meant teleporting or going down stairs or a hole.
The code didn't check for the action return value, and resulted
in a migrating monster being able to throw a potion at hero.

src/monmove.c

@@ -1403,8 +1403,8 @@ m_move(register struct monst* mtmp, register int after)
 
         cnt = mfndpos(mtmp, poss, info, flag);
         if (cnt == 0) {
-            if (find_defensive(mtmp, TRUE))
-                (void) use_defensive(mtmp);
+            if (find_defensive(mtmp, TRUE) && use_defensive(mtmp))
+                return MMOVE_DONE;
             return MMOVE_NOMOVES;
         }
         chcnt = 0;