vxlan: Make vxlan_fdb_find_uc() more robust against NPDs

first_remote_rcu() can return NULL if the FDB entry points to an FDB
nexthop group instead of a remote destination. However, unlike other
users of first_remote_rcu(), NPD cannot currently happen in
vxlan_fdb_find_uc() as it is only invoked by one driver which vetoes the
creation of FDB nexthops.

Make the function more robust by making sure the remote destination is
only dereferenced if it is not NULL.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Wang Liang <wangliang74@huawei.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250908075141.125087-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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