measurement: make sure that DL measurements are valid
measurement: make sure that DL measurements are valid

In repeated_dl_facch_active_decision() we rely on the Measurement
Report message, that in some cases may not contain valid Downlink
measurements.  In this case, we should simply ignore it.

Change-Id: I0d72b454f2b770d5762303fb70a15a8cdc9d07d7
Related: SYS#5114

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