[GUPnP] Output string parameter not initialized on success.

Fix https://github.com/01org/dleyna-renderer/issues/51

Check output string parameters before use, even if the command succeed.

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[GUPnP] Fix bug introduced in previous commit when managing error.

Signed-off-by: Ludovic Ferrandis <ludovic.ferrandis@intel.com>
[Configure] Check for the dbus connector lib availability if required

Signed-off-by: Regis Merlino <regis.merlino@intel.com>