[show] Fix 'show mac' output, when FDB entry with Vlan 1 is present (s…

…onic-net#1368)

* Skip records of FDB entries, which are linked to default Vlan 1,
  to prevent exception throwing while performing
  command 'show mac' or 'fdbshow'.

Signed-off-by: Maksym Belei <Maksym_Belei@jabil.com>

scripts/fdbshow

@@ -87,6 +87,10 @@ class FdbShow(object):
             elif 'bvid' in fdb:
                 try:
                     vlan_id = port_util.get_vlan_id_from_bvid(self.db, fdb["bvid"])
+                    if vlan_id is None:
+                        # the situation could be faced if the system has an FDB entries,
+                        # which are linked to default Vlan(caused by untagged trafic)
+                        continue
                 except Exception:
                     vlan_id = fdb["bvid"]
                     print("Failed to get Vlan id for bvid {}\n".format(fdb["bvid"]))