This change fixes the following panic:
```
2024-06-04T11:16:52.899Z warn app/vmauth/auth_config.go:353 cannot discover backend SRV records for http://srv+localhost:8080: lookup localhost on 10.100.10.4:53: server misbehaving; use it literally
panic: runtime error: integer divide by zero
goroutine 9 [running]:
github.com/VictoriaMetrics/VictoriaMetrics/lib/httpserver.handlerWrapper.func1()
/Users/lhhdz/wd/projects/go/VictoriaMetrics/lib/httpserver/httpserver.go:291 +0x58
panic({0x103115100?, 0x10338d700?})
/Users/lhhdz/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/runtime/panic.go:770 +0x124
main.getLeastLoadedBackendURL({0x0?, 0x22?, 0x1400014757b?}, 0x1400013c120?)
/Users/lhhdz/wd/projects/go/VictoriaMetrics/app/vmauth/auth_config.go:473 +0x210
main.(*URLPrefix).getBackendURL(0x140000aa080)
/Users/lhhdz/wd/projects/go/VictoriaMetrics/app/vmauth/auth_config.go:312 +0xb8
```
---------
Co-authored-by: Haley Wang <haley@victoriametrics.com>
