[iscsi] Ensure SCSI sense data is present before parsing

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/iscsi.c

diff --git a/src/net/tcp/iscsi.c b/src/net/tcp/iscsi.c
index 0d1f0f6..3efee96 100644
--- a/src/net/tcp/iscsi.c
+++ b/src/net/tcp/iscsi.c
@@ -443,7 +443,7 @@
 		rsp.overrun = -(residual_count);
 	}
 	data_len = ISCSI_DATA_LEN ( response->lengths );
-	if ( data_len ) {
+	if ( data_len >= 2 ) {
 		scsi_parse_sense ( ( iscsi->rx_buffer + 2 ), ( data_len - 2 ),
 				   &rsp.sense );
 	}