Fix bug that may cause premature delete of XmppClientConection.

XmppClientConnection::DeleteActor's MayDelete method should not
return true if the XmppClient has no registered event handlers.
It should only look at XmppClientConnection's receiver count.

Change-Id: I6140fd342dd5a4a0ed84c4a140cc3b35a61fed1e
Fix bug that may cause premature delete of XmppClientConection.

Change-Id: Iba1ebc0001b5c1a777debae9c333306b439a44b3