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>