More unpolyable unifying

... and fix the potion dipping case.

include/extern.h

@@ -3154,6 +3154,7 @@ E int FDECL(unturn_dead, (struct monst *));
 E void NDECL(unturn_you);
 E void FDECL(cancel_item, (struct obj *));
 E boolean FDECL(drain_item, (struct obj *, BOOLEAN_P));
+E boolean FDECL(obj_unpolyable, (struct obj *));
 E struct obj *FDECL(poly_obj, (struct obj *, int));
 E boolean FDECL(obj_resists, (struct obj *, int, int));
 E boolean FDECL(obj_shudders, (struct obj *));

src/potion.c

@@ -1977,11 +1977,7 @@ dodip()
             goto poof;
     } else if (obj->otyp == POT_POLYMORPH || potion->otyp == POT_POLYMORPH) {
         /* some objects can't be polymorphed */
-        if (obj->otyp == potion->otyp /* both POT_POLY */
-            || unpolyable(obj)
-            || obj == uball || obj == uskin
-            || obj_resists(obj->otyp == POT_POLYMORPH ? potion : obj,
-                           5, 95)) {
+        if (obj_unpolyable(obj->otyp == POT_POLYMORPH ? potion : obj)) {
             pline1(nothing_happens);
         } else {
             short save_otyp = obj->otyp;

src/zap.c

@@ -1476,6 +1476,16 @@ struct obj *obj;
     delobj(obj);
 }
 
+/* Returns TRUE if obj resists polymorphing */
+boolean
+obj_unpolyable(obj)
+struct obj *obj;
+{
+    return (unpolyable(obj)
+            || obj == uball || obj == uskin
+            || obj_resists(obj, 5, 95));
+}
+
 /* classes of items whose current charge count carries over across polymorph
  */
 static const char charged_objs[] = { WAND_CLASS, WEAPON_CLASS, ARMOR_CLASS,
@@ -1964,7 +1974,7 @@ struct obj *obj, *otmp;
         switch (otmp->otyp) {
         case WAN_POLYMORPH:
         case SPE_POLYMORPH:
-            if (unpolyable(obj) || obj_resists(obj, 5, 95)) {
+            if (obj_unpolyable(obj)) {
                 res = 0;
                 break;
             }