If hci_cmd_sync_complete() is triggered and skb is NULL, then
hdev->req_skb is NULL, which will cause this issue.

Reported-and-tested-by: syzbot+830d9e3fa61968246abd@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>