fix the idle filling to comply with 04.06 5.4.2.3 (UI frame 0 byte len)
fix the idle filling to comply with 04.06 5.4.2.3 (UI frame 0 byte len)

We used to send some crap before, which most phones happily accepted but
some (particularly the Wavecom Q2686) didn't really like at all.

geosphere/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts