enable assertion for debug.

http://ci.rvm.jp/results/trunk-vm-asserts@silicon-docker/2340856

vm_core.h

@@ -491,7 +491,8 @@ rb_iseq_check(const rb_iseq_t *iseq)
 static inline const rb_iseq_t *
 def_iseq_ptr(rb_method_definition_t *def)
 {
-#if VM_CHECK_MODE > 0
+//TODO: re-visit. to check the bug, enable this assertion.
+#if 1 || VM_CHECK_MODE > 0
     if (def->type != VM_METHOD_TYPE_ISEQ) rb_bug("def_iseq_ptr: not iseq (%d)", def->type);
 #endif
     return rb_iseq_check(def->body.iseq.iseqptr);