bug: serpent.UrlOf may cause a panic when parsing url from env

[johnstcn]

It appears that when parsing a URL from the environment, serpent does not dereference it properly when embedded in a struct:

FROBNICATE_URL=http://foo.bar go run main.go 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10083be74]

goroutine 1 [running]:
github.com/coder/serpent.(*URL).Set(0x0, {0x1400001608f?, 0x100849e0a?})
	/Users/cian/go/pkg/mod/github.com/coder/[email protected]/values.go:277 +0x34
github.com/coder/serpent.(*OptionSet).ParseEnv(0x140000ba280, {0x1400027a000, 0x23, 0x10064b324?})
	/Users/cian/go/pkg/mod/github.com/coder/[email protected]/option.go:287 +0x240
github.com/coder/serpent.(*Invocation).run(0x140001d4200, 0x1400025fca8)
	/Users/cian/go/pkg/mod/github.com/coder/[email protected]/command.go:289 +0x58
github.com/coder/serpent.(*Invocation).Run(0x140001d4200)
	/Users/cian/go/pkg/mod/github.com/coder/[email protected]/command.go:536 +0x128
main.main()
	/Users/cian/tmp/serpent-url-panic/main.go:40 +0x268
exit status 2

Sample Go program to reproduce: https://go.dev/play/p/K9uNgbvGM0J

[joobisb]

@johnstcn I did take a look at this.

This could be fixed if we defined it as a value type instead of a pointer. If we use url.URL, it is initialized with its zero value and prevents nil pointer dereference, but if we use a pointer type *url.URL we have to explicitly allocate otherwise it remains nil causing this error.

Something like this, wdyt?

type Options struct {
	URL url.URL
}

{
Name:        "url",
Value:       serpent.URLOf(&o.URL),
 ...                   
}