Fix build on c++ compiler

src/gc.c

@@ -325,7 +325,7 @@ struct RBasic*
 mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
 {
   struct RBasic *p;
-  static const RVALUE RVALUE_zero = { { { 0 } } };
+  static const RVALUE RVALUE_zero = { { { MRB_TT_FALSE } } };
 
 #ifdef MRB_GC_STRESS
   mrb_garbage_collect(mrb);

src/state.c

@@ -53,7 +53,7 @@ mrb_alloca(mrb_state *mrb, size_t size)
 {
   struct alloca_header *p;
 
-  p = mrb_malloc(mrb, sizeof(struct alloca_header)+size);
+  p = (struct alloca_header*) mrb_malloc(mrb, sizeof(struct alloca_header)+size);
   p->next = mrb->mems;
   mrb->mems = p;
   return (void*)p->buf;