meas_rep.c: clang reports a possible division by zero

Check the input and exit early if we have no measurement reports
to avoid a possible division by zero.

openbsc/src/meas_rep.c

@@ -76,6 +76,9 @@ int get_meas_rep_avg(const struct gsm_lchan *lchan,
 	unsigned int i, idx;
 	int avg = 0;
 
+	if (num < 1)
+		return 0;
+
 	idx = calc_initial_idx(ARRAY_SIZE(lchan->meas_rep),
 				lchan->meas_rep_idx, num);