rxrpc: Fix local refcounting

Fix rxrpc_unuse_local() to handle a NULL local pointer as it can be called
on an unbound socket on which rx->local is not yet set.

The following reproduced (includes omitted):

	int main(void)
	{
		socket(AF_RXRPC, SOCK_DGRAM, AF_INET);
		return 0;
	}

causes the following oops to occur:

	BUG: kernel NULL pointer dereference, address: 0000000000000010
	...
	RIP: 0010:rxrpc_unuse_local+0x8/0x1b
	...
	Call Trace:
	 rxrpc_release+0x2b5/0x338
	 __sock_release+0x37/0xa1
	 sock_close+0x14/0x17
	 __fput+0x115/0x1e9
	 task_work_run+0x72/0x98
	 do_exit+0x51b/0xa7a
	 ? __context_tracking_exit+0x4e/0x10e
	 do_group_exit+0xab/0xab
	 __x64_sys_exit_group+0x14/0x17
	 do_syscall_64+0x89/0x1d4
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reported-by: syzbot+20dee719a2e090427b5f@syzkaller.appspotmail.com
Fixes: 730c5fd4 ("rxrpc: Fix local endpoint refcounting")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rxrpc/local_object.c

@@ -402,11 +402,13 @@ void rxrpc_unuse_local(struct rxrpc_local *local)
 {
 	unsigned int au;
 
+	if (local) {
 		au = atomic_dec_return(&local->active_users);
 		if (au == 0)
 			rxrpc_queue_local(local);
 		else
 			rxrpc_put_local(local);
+	}
 }
 
 /*