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
Sponsored by: EMC / Isilon Storage Division
Differential Revision:        https://reviews.freebsd.org/D7041