fix divide by zero

weed/mq/pub_balancer/allocate.go

@@ -69,14 +69,18 @@ func EnsureAssignmentsToActiveBrokers(activeBrokers cmap.ConcurrentMap[string,*B
 	}
 
 	// pick the brokers with the least number of partitions
+	if len(addedAssignments) > 0 {
 		pickedBrokers := pickBrokers(activeBrokers, int32(len(addedAssignments)))
 		for i, assignment := range addedAssignments {
 			assignment.LeaderBroker = pickedBrokers[i]
 		}
-	pickedBrokers = pickBrokers(activeBrokers, int32(len(updatedAssignments)))
+	}
+	if len(updatedAssignments) == 0 {
+		pickedBrokers := pickBrokers(activeBrokers, int32(len(updatedAssignments)))
 		for i, assignment := range updatedAssignments {
 			assignment.LeaderBroker = pickedBrokers[i]
 		}
+	}
 
 	return
 }