sched_lchan_tchh: initialize meas_avg with zeros
sched_lchan_tchh: initialize meas_avg with zeros

in function rx_tchh_fn() the variable meas_avg is not initalized. This
is not always a problem, since most of the time trx_sched_meas_avg() is
populating the variable properly. In cases where a FACCH is transmitted
(chan_state->ul_ongoing_facch = true) the variable is left unpopulated.
In order to have at least stable values for those cases, initalize the
variable with zeros for the ongoing facch phase.

Change-Id: I5c3c1c41d22f9edaaf6bd4478dd04f090dca12a9
Fixes: CID#214480

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts