fib: fix coverity warning/don't dereference NULL

fib_sas6_get is called with NULL dst explicitly so add a NULL check to
avoid a NULL dereference.

Type: fix
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I8ebcba98832f374991f5442c1b83a4b6e64771d0
diff --git a/src/vnet/fib/fib_sas.c b/src/vnet/fib/fib_sas.c
index 8abac86..c9d4693 100644
--- a/src/vnet/fib/fib_sas.c
+++ b/src/vnet/fib/fib_sas.c
@@ -112,7 +112,7 @@
     /*
      * if the dst is v6 and link local, use the source link local
      */
-    if (ip6_address_is_link_local_unicast (dst))
+    if (dst && ip6_address_is_link_local_unicast (dst))
     {
         const ip6_address_t *ll = ip6_get_link_local_address (sw_if_index);
         if (NULL == ll)