ospf6d: Add the missing ospf6 running check in show cmds

SYMPTOM:

If some of the ospfv3 commands like 'show ipv6 ospf6 route' are executed
with ospf6d daemon running but before having any ospfv3 configuration, then
ospf6d crash is seen.

ISSUE:

There are a few show commands, which are (unlike others) not checking if
ospf6 instance is initialized already.

FIX:

Add the missing checks, by using OSPF6_CMD_CHECK_RUNNING() in the commands
where its needed and not yet used.

Signed-off-by: Vipin Kumar <vipin@cumulusnetworks.com>
Reviewed-by: Pradosh Mohapatra <pmohapat@cumulusnetworks.com>
             Dinesh Dutt <ddutt@cumulusnetworks.com>
diff --git a/ospf6d/ospf6_asbr.c b/ospf6d/ospf6_asbr.c
index 6db1ca9..265a178 100644
--- a/ospf6d/ospf6_asbr.c
+++ b/ospf6d/ospf6_asbr.c
@@ -1291,6 +1291,8 @@
 {
   struct ospf6_route *route;
 
+  OSPF6_CMD_CHECK_RUNNING ();
+
   ospf6_redistribute_show_config (vty);
 
   for (route = ospf6_route_head (ospf6->external_table); route;