(mc_serialize_config): fix memory leak.

Found by Coverity.
Coverity id #331818.

Signed-off-by: Andreas Mohr <and@gmx.li>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/serialize.c

@@ -251,6 +251,9 @@ mc_serialize_config (mc_config_t * data, GError ** error)
 
         g_strfreev (params);
     }
+
+    g_strfreev (groups);
+
     return g_string_free (buffer, FALSE);
 }