iscsi-target: avoid NULL pointer in iscsi_copy_param_list failure

In iscsi_copy_param_list() a failed iscsi_param_list memory allocation
currently invokes iscsi_release_param_list() to cleanup, and will promptly
trigger a NULL pointer dereference.

Instead, go ahead and return for the first iscsi_copy_param_list()
failure case.

Found by coverity.

Signed-off-by: Joern Engel <joern@logfs.org>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>