fix debug build bustage from Bug 559408.

js/src/jsarena.h

@@ -241,7 +241,9 @@ class JSArenaPool {
      * |p| must be |a->base| for the arena |a| that contains |p|.
      */
     JSArena ***ptrToHeader(void *p) const {
+#ifdef JS_ALIGN_OF_POINTER
         JS_ASSERT((jsuword(p) & headerBaseMask()) == 0);
+#endif
         return (JSArena ***)(p) - 1;
     }