xfreerdp: fixed crash with --authonly

Also updated documentation: --authonly returns 0 on success
and != 0 if an error occurs.

fixes #843

client/X11/xfreerdp.1.xml

@@ -300,7 +300,7 @@
         <listitem>
           <para>
             Only authenticates. This is useful to test your credentials (username and password).
-						Returns status code 0 if the client can connect, 1 otherwise. Requires a username,
+						Returns status code 0 if the client can connect. Requires a username,
 						password and connection host at the command line.
           </para>
         </listitem>

client/X11/xfreerdp.c

@@ -636,6 +636,9 @@ boolean xf_post_connect(freerdp* instance)
 	cache = instance->context->cache;
 	channels = xfi->_context->channels;
 
+	if (instance->settings->authentication_only)
+		return true;
+
 	if (xf_get_pixmap_info(xfi) != true)
 		return false;
 
@@ -976,6 +979,13 @@ int xfreerdp_run(freerdp* instance)
 	if (!freerdp_connect(instance))
 		return XF_EXIT_CONN_FAILED;
 
+	if (instance->settings->authentication_only)
+	{
+		freerdp_disconnect(instance);
+		freerdp_free(instance);
+		return ret;
+	}
+
 	xfi = ((xfContext*) instance->context)->xfi;
 	channels = instance->context->channels;