[drouting] fixed weight based selection

if the last element has weight 1, it will never be selected.
This was instroduced with 48c051f.
The fix is to actually follow the initial PR approach, which was correct. My attempt to improve the original PR actually broke stuff there :(

modules/drouting/drouting.c

@@ -2982,11 +2982,11 @@ static int weight_based_sort(pgw_list_t *pgwl, int size, unsigned short *idx)
 		}
 		if (weight_sum) {
 			/* randomly select number */
-			rand_no = (unsigned int)(weight_sum*((double)rand()/((double)RAND_MAX)));
+			rand_no = (unsigned int)(weight_sum*((double)rand()/((double)1+RAND_MAX)));
 			LM_DBG("random number is %d\n",rand_no);
 			/* select the element */
 			for( i=first ; i<size ; i++ )
-				if (running_sum[i]>=rand_no) break;
+				if (running_sum[i]>rand_no) break;
 			if (i==size) {
 				LM_CRIT("bug in weight sort, first=%u, size=%u, rand_no=%u, total weight=%u\n",
 					first, size, rand_no, weight_sum);