Improve wand of nothing randomizing

As per Sean's suggestion, just use rn2. This means the line
can also be moved to better place, with the other object
init stuff.

src/allmain.c

@@ -559,7 +559,6 @@ newgame()
     init_artifacts(); /* before u_init() in case $WIZKIT specifies
                        * any artifacts */
     u_init();
-    objects[WAN_NOTHING].oc_dir = (ubirthday % 2L) ? NODIR : IMMEDIATE;
 
 #ifndef NO_SIGNAL
     (void) signal(SIGINT, (SIG_RET_TYPE) done1);

src/o_init.c

@@ -178,6 +178,7 @@ init_objects()
 #ifdef USE_TILES
     shuffle_tiles();
 #endif
+    objects[WAN_NOTHING].oc_dir = rn2(2) ? NODIR : IMMEDIATE;
 }
 
 /* retrieve the range of objects that otyp shares descriptions with */