dccp ccid-3: Replace lazy BUG_ON with condition

The BUG_ON(w_tot == 0) only holds if there is no more than 1 loss interval in
the loss history. If there is only a single loss interval, the calc_i_mean()
routine need in fact not be called (RFC 3448, 6.3.1). 
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

net/dccp/ccids/lib/loss_interval.c

@@ -67,7 +67,10 @@ static void tfrc_lh_calc_i_mean(struct tfrc_loss_hist *lh)
 	u32 i_i, i_tot0 = 0, i_tot1 = 0, w_tot = 0;
 	int i, k = tfrc_lh_length(lh) - 1; /* k is as in rfc3448bis, 5.4 */
 
-	for (i=0; i <= k; i++) {
+	if (k <= 0)
+		return;
+
+	for (i = 0; i <= k; i++) {
 		i_i = tfrc_lh_get_interval(lh, i);
 
 		if (i < k) {
@@ -78,7 +81,6 @@ static void tfrc_lh_calc_i_mean(struct tfrc_loss_hist *lh)
 			i_tot1 += i_i * tfrc_lh_weights[i-1];
 	}
 
-	BUG_ON(w_tot == 0);
 	lh->i_mean = max(i_tot0, i_tot1) / w_tot;
 }