dialogkeeper: avoid crash caused by deletion inside finished signal (refs #2611)
Since 47028d1bd97d, MergeDialog causes segfault if closed by "Finish" button. This is because MergeDialog doesn't reimplement accept() in Python, so it can be garbage-collected once DialogKeeper removes a reference.
DialogKeeper shouldn't change refcount of finished dialog inside signal handler.
This change shadows the exception reported by #2611.