Protected `proc` objects given by `OP_DEF` on registers
If a GC occurs in `mrb_calloc()` called by `mt_rehash()` inside `mrb_define_method_raw(), the `proc` may be destroyed.
@@ -3763,6 +3763,7 @@ codegen(codegen_scope *s, node *tree, int val)
genop_1(s, OP_SCLASS, cursp());
push();
genop_2(s, OP_METHOD, cursp(), idx);
+ push(); pop();
pop();
genop_2(s, OP_DEF, cursp(), sym);
if (val) push();
