Call shutdown() in connect() on exception

to clean up the state build up by connect().

Related to SMACK-855 there is the possiblitiy of a stray (writer)
thread if, for example, tlsHandled.checkifSuccessOrWaitorThrow() in
XMPPTCPConnection.connectInternal() throws. This commit should prevent
that.

smack-core/src/test/java/org/jivesoftware/smack/DummyConnection.java

@@ -100,6 +100,11 @@ public class DummyConnection extends AbstractXMPPConnection {
         callConnectionClosedListener();
     }
 
+    @Override
+    public void instantShutdown() {
+        shutdown();
+    }
+
     @Override
     public boolean isSecureConnection() {
         return false;
@@ -226,4 +231,5 @@ public class DummyConnection extends AbstractXMPPConnection {
             }
         }
     }
+
 }