Bluetooth: SMP: Fix variable dereferenced before check 'conn'

This fixes kbuild findings:

smatch warnings:
net/bluetooth/smp.c:1633 smp_user_confirm_reply() warn: variable
dereferenced before check 'conn' (see line 1631)

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>