MFC r208512:   sge_encap() can sometimes return an error with m_head set to NULL.   Make sure not to requeue freed mbuf in sge_start_locked(). This   should fix NULL pointer dereference panic.
MFC r208512:
  sge_encap() can sometimes return an error with m_head set to NULL.
  Make sure not to requeue freed mbuf in sge_start_locked(). This
  should fix NULL pointer dereference panic.

  Reported by:	Nikolay Denev <ndenev <> gmail dot com>
  Submitted by:	jhb
  Approved by:	re (bz)

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