power_control: properly track the first initial state

Change-Id: I16e7474b5affbd90855a2e407b305e9dec581dfa
Related: SYS#4918, SYS#4917

include/osmo-bts/gsm_data.h

@@ -215,6 +215,8 @@ extern const struct gsm_power_ctrl_params power_ctrl_params_def;
 
 /* Measurement pre-processing state */
 struct gsm_power_ctrl_meas_proc_state {
+	/* Number of measurements processed */
+	unsigned int meas_num;
 	/* Algorithm specific data */
 	union {
 		struct {

src/common/power_control.c

@@ -78,7 +78,7 @@ static int do_pf_ewma(const struct gsm_power_ctrl_meas_params *mp,
 	int *Avg100 = &mps->ewma.Avg100;
 
 	/* We don't have 'Avg[n - 1]' if this is the first run */
-	if (*Avg100 == 0) {
+	if (mps->meas_num++ == 0) {
 		*Avg100 = Val * EWMA_SCALE_FACTOR;
 		return Val;
 	}