msg: Remove unnecessary call of nlmsg_free on known NULL pointer

In nlmsg_convert, if __nlmsg_alloc fails we can return NULL directly
instead of unnecessarily calling nlmsg_free on the NULL pointer.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>