Replace 0 with NULL because set NULL in sturct pointer.

src/gc.c

@@ -312,8 +312,8 @@ add_heap(mrb_state *mrb)
 void
 mrb_init_heap(mrb_state *mrb)
 {
-  mrb->heaps = 0;
-  mrb->free_heaps = 0;
+  mrb->heaps = NULL;
+  mrb->free_heaps = NULL;
   add_heap(mrb);
   mrb->gc_interval_ratio = DEFAULT_GC_INTERVAL_RATIO;
   mrb->gc_step_ratio = DEFAULT_GC_STEP_RATIO;
@@ -588,7 +588,7 @@ obj_free(mrb_state *mrb, struct RBasic *obj)
 
       if (e->cioff < 0) {
         mrb_free(mrb, e->stack);
-        e->stack = 0;
+        e->stack = NULL;
       }
     }
     break;
@@ -643,8 +643,8 @@ root_scan_phase(mrb_state *mrb)
   size_t i, e, j;
 
   if (!is_minor_gc(mrb)) {
-    mrb->gray_list = 0;
-    mrb->variable_gray_list = 0;
+    mrb->gray_list = NULL;
+    mrb->variable_gray_list = NULL;
   }
 
   mrb_gc_mark_gv(mrb);
@@ -778,7 +778,7 @@ final_marking_phase(mrb_state *mrb)
   }
   gc_assert(mrb->gray_list == NULL);
   mrb->gray_list = mrb->variable_gray_list;
-  mrb->variable_gray_list = 0;
+  mrb->variable_gray_list = NULL;
   while (mrb->gray_list) {
     if (is_gray(mrb->gray_list))
       gc_mark_children(mrb, mrb->gray_list);