The code-path which checks whether the server requires an "xmpp-session"
was untested and therefore the connection-callback handler was triggered
twice with the state `XMPP_CONN_CONNECT`. This happened if the server
supports stream-management and requires a session.
Reported via https://github.com/profanity-im/profanity/issues/1954
Fixup of c7d410f38b
Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>