Make dl_list_first() and dl_list_last() uses easier for static analyzers

Make dl_list_first() and dl_list_last() uses easier for static analyzers

The previous check for dl_list_len() or having an entry from the list is
sufficient, but some static analyzers cannot figure out that
dl_list_first() and dl_list_last() will return non-NULL in this type of
cases. Avoid invalid reports by explicitly checking for NULL.

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