Bluetooth: Check 'dev_class' in mgmt_device_found()

The mgmt_device_found event will be used to report LE devices found
during discovery procedure. Since LE advertising reports events
doesn't have class of device information, we need to check if
'dev_class' is not NULL before copying it.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>