libgupnp-dlna: Fix relaxed-mode <field> processing
libgupnp-dlna: Fix relaxed-mode <field> processing

When skipping a field, we need to make sure we don't touch the out
parameter (caps_str).

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