Bluetooth: Fix null pointer deref on unexpected status event

__hci_cmd_sync returns NULL if the controller responds with a status
event. This is unexpected for the commands sent here, but on
occurrence leads to null pointer dereferences and thus must be
handled.

Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>