revert part of #507 for warning

src/gc.c

@@ -326,7 +326,6 @@ struct RBasic*
 mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
 {
   struct RBasic *p;
-  static const RVALUE RVALUE_zero = { 0 };
 
 #ifdef MRB_GC_STRESS
   mrb_garbage_collect(mrb);
@@ -346,7 +345,7 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
 
   mrb->live++;
   gc_protect(mrb, p);
-  *(RVALUE *)p = RVALUE_zero;
+  memset(p, 0, sizeof(RVALUE));
   p->tt = ttype;
   p->c = cls;
   paint_partial_white(mrb, p);