Fix crash in SessionPool::onPasswordRequestDone, again due to m_pendingInitialSession.

It could become dangling in the following case:
- wrong imap password, error dialog shows up, leave it up
- wait for connection to drop (idle timeout, or maybe due to losing network altogether)
- resource reconnects, wrong password still, error dialog show up.
When closing them both, the 2nd one ends up in SessionPool::onPasswordRequestDone with
a dangling m_pendingInitialSession.