tcm_fc: Check for aborted sequence

Add a check for an aborted sequence, which has a
NULL sequence pointer, to avoid target crashes.
The most relevant messages from the crash (entered
from video capture) include:

BUG: unable to handle kernel paging request at ffffffffffffffdf
IP: [<ffffffffa02d514c>] fc_seq_send+0x3c/0x150 [libfc]
...
Call Trace:
 [<ffffffffa0443de6>] ft_queue_data_in+0x266/0x560 [tcm_fc]

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>