iscsi-target: remove dead code in iscsi_check_valuelist_for_support

Neither "acceptor_values" nor "proposer_values" can be NULL here when
scanning the value lists for incoming iSCSI login parameters such as
HeaderDigest=CRC32C,None.

Smatch complains because we are not allowed to pass NULL pointers to
strchr().  Also I removed a second later check for "!acceptor_values"
because it gets checked on the next line in the do while condition.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>