libgupnp-dlna: only free format caps if present
libgupnp-dlna: only free format caps if present

This gets rid of a GStreamer warning in certain cases when we try to
free NULL caps.

GNOME/gupnp-dlna,GNOME/gupnp-dlna,luisbg/gupnp-dlna,GNOME/gupnp-dlna,luisbg/gupnp-dlna