Bluetooth: HCI: Fix potential null-ptr-deref

Fix potential null-ptr-deref in hci_le_big_sync_established_evt().

Fixes: f777d882 (Bluetooth: ISO: Notify user space about failed bis connections)
Signed-off-by: Sungwoo Kim <iam@sung-woo.kim>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>