Fix crystal ball use after being destroyed

Crystal ball can be destroyed when used, so pass the object parameter
back as null in that case.

include/extern.h

@@ -243,7 +243,7 @@ E int FDECL(object_detect, (struct obj *, int));
 E int FDECL(monster_detect, (struct obj *, int));
 E int FDECL(trap_detect, (struct obj *));
 E const char *FDECL(level_distance, (d_level *));
-E void FDECL(use_crystal_ball, (struct obj *));
+E void FDECL(use_crystal_ball, (struct obj **));
 E void NDECL(do_mapping);
 E void NDECL(do_vicinity_map);
 E void FDECL(cvt_sdoor_to_door, (struct rm *));

src/apply.c

@@ -3486,7 +3486,7 @@ doapply()
         res = use_towel(obj);
         break;
     case CRYSTAL_BALL:
-        use_crystal_ball(obj);
+        use_crystal_ball(&obj);
         break;
     case MAGIC_MARKER:
         res = dowrite(obj);

src/artifact.c

@@ -1401,13 +1401,13 @@ doinvoke()
 
 STATIC_OVL int
 arti_invoke(obj)
-register struct obj *obj;
+struct obj *obj;
 {
     register const struct artifact *oart = get_artifact(obj);
 
     if (!oart || !oart->inv_prop) {
         if (obj->otyp == CRYSTAL_BALL)
-            use_crystal_ball(obj);
+            use_crystal_ball(&obj);
         else
             pline1(nothing_happens);
         return 1;

src/detect.c

@@ -891,11 +891,12 @@ static const struct {
 };
 
 void
-use_crystal_ball(obj)
-struct obj *obj;
+use_crystal_ball(optr)
+struct obj **optr;
 {
     char ch;
     int oops;
+    struct obj *obj = *optr;
 
     if (Blind) {
         pline("Too bad you can't see %s.", the(xname(obj)));
@@ -930,7 +931,7 @@ struct obj *obj;
         case 5:
             pline("%s!", Tobjnam(obj, "explode"));
             useup(obj);
-            obj = 0; /* it's gone */
+            *optr = obj = 0; /* it's gone */
             /* physical damage cause by the shards and force */
             losehp(Maybe_Half_Phys(rnd(30)), "exploding crystal ball",
                    KILLED_BY_AN);