Suppress warning (uninitialized variable).

gc.c

@@ -1088,7 +1088,7 @@ check_rvalue_consistency_force(const VALUE obj, int terminate)
     }
     else if (!is_pointer_to_heap(objspace, (void *)obj)) {
         /* check if it is in tomb_pages */
-        struct heap_page *page;
+        struct heap_page *page = NULL;
         list_for_each(&heap_tomb->pages, page, page_node) {
             if (&page->start[0] <= (RVALUE *)obj &&
                 (RVALUE *)obj < &page->start[page->total_slots]) {