XEP-0198: Don't drop session on failed resume

The `previd` value provided by the client during a session resume request includes the client's JID and ejabberd's session ID.  If there is a session for the requested JID but with a different session ID, resumption should fail, but that session shouldn't be closed.  This commit makes sure the latter won't happen.

In practice, this will only make a difference in odd corner cases.