Using non-existent labels placeholders will cause errors

[nothinggift]

community link

CaddyFile:

:80 {
  encode zstd gzip
  file_server
  try_files {path} /index.html
  root * /data/web/{labels.2}/dist/{labels.3}
}

If labels.3 does not exist, it will cause an error

2020/06/16 17:16:02 http: panic serving 223.166.222.111:63424: runtime error: index out of range [-1]
goroutine 1971 [running]:
net/http.(*conn).serve.func1(0xc000a0bd60)
	net/http/server.go:1772 +0x139
panic(0x1700340, 0xc0011d6ec0)
	runtime/panic.go:975 +0x3e3
github.com/caddyserver/caddy/v2/modules/caddyhttp.addHTTPVarsToReplacer.func1(0xc000d3ab6d, 0x1a, 0x0, 0x0, 0x0)
	github.com/caddyserver/caddy/v2@v2.1.0-beta.1/modules/caddyhttp/replacer.go:180 +0x216f
github.com/caddyserver/caddy/v2.(*Replacer).Get(0xc0011a0640, 0xc000d3ab6d, 0x1a, 0x1, 0x1b, 0xc000ce1c01)
	github.com/caddyserver/caddy/v2@v2.1.0-beta.1/replacer.go:62 +0x77
github.com/caddyserver/caddy/v2.(*Replacer).replace(0xc0011a0640, 0xc000d3ab40, 0x48, 0x0, 0x0, 0x1, 0x0, 0xc0009eac80, 0x4, 0x0, ...)
	github.com/caddyserver/caddy/v2@v2.1.0-beta.1/replacer.go:164 +0x3a7
github.com/caddyserver/caddy/v2.(*Replacer).ReplaceAll(...)
	github.com/caddyserver/caddy/v2@v2.1.0-beta.1/replacer.go:101

[mholt]

Thanks for the report!