WPS UPnP: Make dl_list_first() use easier for static analyzer

WPS UPnP: Make dl_list_first() use easier for static analyzer

The preceding dl_list_len() check guarantees that dl_list_first()
returns an entry and not NULL. However, that seems to be a bit too
difficult path to follow for static analyzers, so add an explicit check
for the dl_list_first() return value to be non-NULL to avoid warnings.

Signed-off-by: Jouni Malinen <j@w1.fi>