Fix invalid enqueue of NULL in bxe tx path

[rstone]

Summary

bxe could incorrectly enqueue NULL on its buf_ring. Most places
in the driver explicitly test for this case and skip it the enqueue
when the mbuf, but one case missed it.

Test Plan

With INVARIANTS enabled, ran a test that stressed the tx path. Under
an overload condition NULL could be enqueued twice to the buf_ring,
leading to an kassert firing.

[davidcs]

Looks Good