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/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 */