isisd: fix an error that was probably a result of copypasting

The code should check for the existance of the correct list prior to
accessing it.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/isisd/isis_circuit.c b/isisd/isis_circuit.c
index da0b57a..1d1a59e 100644
--- a/isisd/isis_circuit.c
+++ b/isisd/isis_circuit.c
@@ -975,7 +975,7 @@
               vty_out(vty, "      %s%s", buf, VTY_NEWLINE);
             }
         }
-      if (circuit->ipv6_link && listcount(circuit->ipv6_non_link) > 0)
+      if (circuit->ipv6_non_link && listcount(circuit->ipv6_non_link) > 0)
         {
           vty_out(vty, "    IPv6 Prefixes:%s", VTY_NEWLINE);
           for (ALL_LIST_ELEMENTS_RO(circuit->ipv6_non_link, node, ip_addr))