Bluetooth: Fix potential NULL dereference

The bnep_get_device function may be triggered by an ioctl just after a
connection has gone down. In such a case the respective L2CAP chan->conn
pointer will get set to NULL (by l2cap_chan_del). This patch adds a
missing NULL check for this case in the bnep_get_device() function.

Reported-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>