Possible division by zero

[Bbulatov]

Hello!
During the static analysis was found possible mistake:

1. Variable subConnsLen, whose possible value set allows a zero value at roundrobin.go:76, is used as a denominator at roundrobin.go:79.

func (p *rrPicker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
	subConnsLen := uint32(len(p.subConns))
	nextIndex := atomic.AddUint32(&p.next, 1)

	sc := p.subConns[nextIndex%subConnsLen]
	return balancer.PickResult{SubConn: sc}, nil
}

version of gRPC: master
version of Go: go version go1.22.4 linux/amd64
operating system: Linux debian 10

[aranjans]

@Bbulatov This case is already handled by base balancer here and hence we won't need to have this condition in roundrobin explicitly.