Don't enqueue NULL on a drbr

In one corner case in the bxe TX path, a NULL mbuf could be enqueued onto
a drbr queue. This could case a KASSERT to fire with INVARIANTS enabled,
or the processing of packets from the queue to be prematurely ended later
on.

Submitted by: Matt Joras (matt.joras AT isilon.com)
Reviewed by: davidcs
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7041