Bluetooth: Fix potential NULL pointer dereference in smp_conn_security

The smp pointer might not be initialized for jumps to the "done" label
in the smp_conn_security function. Furthermore doing the set_bit after
done might "overwrite" a previous value of the flag in case pairing was
already in progress. This patch moves the call to set_bit before the
label so that it is only done for a newly created smp context (as
returned by smp_chan_create).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>