Move TLS Required check at the end of connect()

It was a *very* bad idea to perform the SecurityMode.Required check in
the connection's reader thread and not at the end of
AbstractXMPPConnectin's connect(). :/

This behavior dates back to 8e750912a7

Fixes SMACK-739

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -362,6 +362,7 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
 
         // Perform the actual connection to the XMPP service
         connectInternal();
+
         return this;
     }