ipsec: Fix NULL encryption algorithm

Type: fix
Ticket: VPP-1756

the block-size was set to 0 resulting in incorrect placement of the ESP
footer.

add tests for NULL encrypt + integ.

Change-Id: I8ab3afda8e68f9ff649540cba3f2cac68f12bbba
Signed-off-by: Neale Ranns <nranns@cisco.com>

test/template_ipsec.py

@@ -379,7 +379,7 @@ class IpsecTra4(object):
 
         # a malformed 'runt' packet
         #  created by a mis-constructed SA
-        if (ESP == self.encryption_type):
+        if (ESP == self.encryption_type and p.crypt_algo != "NULL"):
             bogus_sa = SecurityAssociation(self.encryption_type,
                                            p.vpp_tra_spi)
             pkt = (Ether(src=self.tra_if.remote_mac,