fib_trie: Avoid NULL pointer if local table is not allocated

The function fib_unmerge assumed the local table had already been
allocated.  If that is not the case however when custom rules are applied
then this can result in a NULL pointer dereference.

In order to prevent this we must check the value of the local table pointer
and if it is NULL simply return 0 as there is no local table to separate
from the main.

Fixes: 0ddcf43d5 ("ipv4: FIB Local/MAIN table collapse")
Reported-by: Madhu Challa <challa@noironetworks.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -156,9 +156,12 @@ int fib_unmerge(struct net *net)
 {
 	struct fib_table *old, *new;
 
+	/* attempt to fetch local table if it has been allocated */
 	old = fib_get_table(net, RT_TABLE_LOCAL);
-	new = fib_trie_unmerge(old);
+	if (!old)
+		return 0;
 
+	new = fib_trie_unmerge(old);
 	if (!new)
 		return -ENOMEM;