Bluetooth: Fix missing NULL check for smp_chan_create() return value

Bluetooth: Fix missing NULL check for smp_chan_create() return value

The smp_chan_create function may return NULL, e.g. in the case of memory
allocation failure, so we always need to check for this.

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