net/http: ProxyFromEnvironment panics on malformed no_proxy contents

[calmh]

In Go 1.8, the following example program:

package main

import (
	"fmt"
	"net/http"
)

func main() {
	cli := http.Client{
		Transport: &http.Transport{
			Proxy: http.ProxyFromEnvironment,
		},
	}
	_, err := cli.Get("https://google.com/")
	if err != nil {
		fmt.Println(err)
	}
}

panics on a no_proxy containing a malformed entry:

$ no_proxy=:1 http_proxy=http://127.0.0.1:1234 go run main.go 
panic: runtime error: index out of range

goroutine 1 [running]:
net/http.useProxy(0xc420011100, 0xa, 0xe)
	/usr/local/go/src/net/http/transport.go:1179 +0x48b
net/http.ProxyFromEnvironment(0xc42000a800, 0xc4200110f0, 0xe, 0x18)
	/usr/local/go/src/net/http/transport.go:274 +0xd1
net/http.(*Transport).connectMethodForRequest(0xc4200dc000, 0xc4200ca7e0, 0x0, 0x1263948, 0x5, 0xc4200110f0, 0xe, 0x10, 0xc42004ba50)
	/usr/local/go/src/net/http/transport.go:581 +0x14b
net/http.(*Transport).RoundTrip(0xc4200dc000, 0xc42000a800, 0xc4200dc000, 0x0, 0x0)
	/usr/local/go/src/net/http/transport.go:368 +0x4e6
net/http.send(0xc42000a800, 0x1354b20, 0xc4200dc000, 0x0, 0x0, 0x0, 0x0, 0x8, 0xc42000e048, 0x1010c58)
	/usr/local/go/src/net/http/client.go:249 +0x162
net/http.(*Client).send(0xc420018ea0, 0xc42000a800, 0x0, 0x0, 0x0, 0xc42000e048, 0x0, 0x1, 0xc420018ed0)
	/usr/local/go/src/net/http/client.go:173 +0x108
net/http.(*Client).Do(0xc420018ea0, 0xc42000a800, 0x1263948, 0x13, 0x0)
	/usr/local/go/src/net/http/client.go:595 +0x254
net/http.(*Client).Get(0xc420018ea0, 0x1263948, 0x13, 0xc420018d80, 0xc420018cf0, 0x0)
	/usr/local/go/src/net/http/client.go:393 +0xa7
main.main()
	/Users/jb/src/github.com/calmh/junk/proxypanic/main.go:14 +0xfe
exit status 2

The :1 in no_proxy should have been ::1, but a panic seems inappropriate in any case. Expected results would have been for the request to be proxied (as it did not match any no_proxy entries) or, possibly, return an error indicating that the proxy configuration couldn't be determined.

go version go1.8 darwin/amd64, same problem also present in go1.7.4 so it's not a regression in 1.8.

[gopherbot]

CL https://golang.org/cl/38067 mentions this issue.