Don't request registration forms unless the register event is handled.

Some servers end the stream if registration can not be completed
in-band, which means always requesting the form can prevent regular
login.

sleekxmpp/plugins/xep_0077/register.py

@@ -50,7 +50,7 @@ class XEP_0077(BasePlugin):
             # We have already logged in with an account
             return False
 
-        if self.create_account:
+        if self.create_account and self.xmpp.event_handled('register'):
             form = self.get_registration()
             self.xmpp.event('register', form, direct=True)
             return True