fix github issue #106 - polymorph panic

Fixes #106

If dipping a worn amulet into a potion of polymorph turns it into an
amulet of change, the game panics while trying to use up that amulet
when the new one hasn't replaced the old one in inventory yet.  Simply
reordering the relevant code isn't sufficient to fix things:  once it
is in inventory and can be successfully used up, later code would end
up deferencing a stale pointer because it was unaware of the deletion.

src/potion.c

@@ -1920,26 +1920,22 @@ dodip()
                            5, 95)) {
             pline1(nothing_happens);
         } else {
-            boolean was_wep, was_swapwep, was_quiver;
             short save_otyp = obj->otyp;
 
             /* KMH, conduct */
             u.uconduct.polypiles++;
 
-            was_wep = (obj == uwep);
-            was_swapwep = (obj == uswapwep);
-            was_quiver = (obj == uquiver);
-
             obj = poly_obj(obj, STRANGE_OBJECT);
 
-            if (was_wep)
-                setuwep(obj);
-            else if (was_swapwep)
-                setuswapwep(obj);
-            else if (was_quiver)
-                setuqwep(obj);
-
-            if (obj->otyp != save_otyp) {
+            /*
+             * obj might be gone:
+             *  poly_obj() -> set_wear() -> Amulet_on() -> useup()
+             * if obj->otyp is worn amulet and becomes AMULET_OF_CHANGE.
+             */
+            if (!obj) {
+                makeknown(POT_POLYMORPH);
+                return 1;
+            } else if (obj->otyp != save_otyp) {
                 makeknown(POT_POLYMORPH);
                 useup(potion);
                 prinv((char *) 0, obj, 0L);