ifp->if_transmit() must free mbuf even if error occurred.

Add missing m_freem(m) to if_nulltransmit().
Below ifp->if_transmit() implementations are already added m_freem(m) properly.
    - wm(4)
    - ixg(4)
    - ixv(4)
    - pppoe(4)
    - gif(4)
    - l2tp(4)

pointed out by ozaki-r@n.o, thanks.

(knakahara)