ipv6: check fn before doing FIB6_SUBTREE(fn)

In fib6_locate(), we need to first make sure fn is not NULL before doing
FIB6_SUBTREE(fn) to avoid crash.

This fixes the following static checker warning:
net/ipv6/ip6_fib.c:1462 fib6_locate()
         warn: variable dereferenced before check 'fn' (see line 1459)

net/ipv6/ip6_fib.c
  1458          if (src_len) {
  1459                  struct fib6_node *subtree = FIB6_SUBTREE(fn);
                                                    ^^^^^^^^^^^^^^^^
We shifted this dereference

  1460
  1461                  WARN_ON(saddr == NULL);
  1462                  if (fn && subtree)
                            ^^
before the check for NULL.

  1463                          fn = fib6_locate_1(subtree, saddr, src_len,
  1464                                             offsetof(struct rt6_info, rt6i_src)

Fixes: 66f5d6ce ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>