In case of make_ll64 fails return

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>

nfdhcpd

@@ -283,6 +283,8 @@ class Subnet(object):
         """ Compute an EUI-64 address from an EUI-48 (MAC) address
 
         """
+        if mac is None:
+            return None
         comp = mac.split(":")
         prefix = IPy.IP(net).net().strFullsize().split(":")[:4]
         eui64 = comp[:3] + ["ff", "fe"] + comp[3:]
@@ -681,6 +683,8 @@ class VMNetProxy(object): # pylint: disable=R0902
 
         indevmac = self.get_iface_hw_addr(binding.indev)
         ifll = subnet.make_ll64(indevmac)
+        if ifll is None:
+            return
 
 
         resp = Ether(src=indevmac)/\
@@ -742,6 +746,8 @@ class VMNetProxy(object): # pylint: disable=R0902
         indevmac = self.get_iface_hw_addr(binding.indev)
 
         ifll = subnet.make_ll64(indevmac)
+        if ifll is None:
+            return
 
         if not (subnet.net.overlaps(ns.tgt) or str(ns.tgt) == str(ifll)):
             logging.debug("Received NS for a non-routable IP (%s)", ns.tgt)
@@ -783,6 +789,8 @@ class VMNetProxy(object): # pylint: disable=R0902
                 continue
             indevmac = self.get_iface_hw_addr(indev)
             ifll = subnet.make_ll64(indevmac)
+            if ifll is None:
+                continue
             resp = Ether(src=indevmac)/\
                    IPv6(src=str(ifll))/ICMPv6ND_RA(routerlifetime=14400)/\
                    ICMPv6NDOptPrefixInfo(prefix=str(subnet.prefix),