Don't compare the target methods for SlotObject connections

When a new-style connection is created (a SlotObject), we don't store
the method offset since there isn't one. So don't try to read it.

Qt::UniqueConnection only applies to old-style connections, since we
can't compare the slot objects for equality. In any case, an old-style
connection and a new style will never be considered equal.

Task-number: QTBUG-39927
Change-Id: I10a39a7bc97a2ec9509a0708038cc491bcc67329
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Elias Probst <mail@eliasprobst.eu>

src/corelib/kernel/qobject.cpp

@@ -3230,7 +3230,7 @@ QObjectPrivate::Connection *QMetaObjectPrivate::connect(const QObject *sender,
             int method_index_absolute = method_index + method_offset;
 
             while (c2) {
-                if (c2->receiver == receiver && c2->method() == method_index_absolute)
+                if (!c2->isSlotObject && c2->receiver == receiver && c2->method() == method_index_absolute)
                     return 0;
                 c2 = c2->nextConnectionList;
             }