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.

mrbgems/mruby-compiler/core/codegen.c

@@ -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();