Assert not to be UNDEF visibility

Any defined methods, bound to any classes/modules and not being
UNDEFINED_METHOD_ENTRY_P, should not be METHOD_VISI_UNDEF.

class.c

@@ -1435,6 +1435,7 @@ method_entry_i(ID key, VALUE value, void *data)
 	}
 	else {
 	    type = METHOD_ENTRY_VISI(me);
+	    RUBY_ASSERT(type != METHOD_VISI_UNDEF);
 	}
 	st_add_direct(arg->list, key, (st_data_t)type);
     }

proc.c

@@ -1657,6 +1657,7 @@ mnew_internal(const rb_method_entry_t *me, VALUE klass, VALUE iclass,
     }
     if (visi == METHOD_VISI_UNDEF) {
 	visi = METHOD_ENTRY_VISI(me);
+	RUBY_ASSERT(visi != METHOD_VISI_UNDEF); /* !UNDEFINED_METHOD_ENTRY_P(me) */
 	if (scope && (visi != METHOD_VISI_PUBLIC)) {
 	    if (!error) return Qnil;
 	    rb_print_inaccessible(klass, id, visi);