Bluetooth: Fix NULL pointer deference on eir_get_service_data

The len parameter is considered optional so it can be NULL so it cannot
be used for skipping to next entry of EIR_SERVICE_DATA.

Fixes: 8f9ae5b3 ("Bluetooth: eir: Add helpers for managing service data")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>