panic on nil *bytes.Buffer as body

[alexdulin]

When NewRequestWithContext is given a nil *bytes.Buffer as the body, it panics. Best guess is that it is because the type of the body is being checked, but not that it is non-nil.

Reproduction code (example here):

package main

import (
	"bytes"
	"context"
	"net/http"

	"github.com/hashicorp/go-retryablehttp"
)

type config struct {
	Method string
	URL    string
	Body   *bytes.Buffer
}

func main() {
	cfg := config{
		Method: http.MethodGet,
		URL:    "https://example.com",
	}
	_, _ = retryablehttp.NewRequestWithContext(
		context.Background(),
		cfg.Method,
		cfg.URL,
		cfg.Body,
	)
}

Panic output:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x5edb1c]

goroutine 1 [running]:
bytes.(*Buffer).Len(...)
	/usr/local/go-faketime/src/bytes/buffer.go:73
github.com/hashicorp/go-retryablehttp.getBodyReaderAndContentLength({0x63e680?, 0x0?})
	/tmp/gopath1053138375/pkg/mod/github.com/hashicorp/go-retryablehttp@v0.7.2/client.go:228 +0x35c
github.com/hashicorp/go-retryablehttp.NewRequestWithContext({0x6afd98, 0xc0000ac000}, {0x648621, 0x3}, {0x64c514, 0x13}, {0x63e680?, 0x0?})
	/tmp/gopath1053138375/pkg/mod/github.com/hashicorp/go-retryablehttp@v0.7.2/client.go:295 +0x5a
main.main()
	/tmp/sandbox1468963018/prog.go:22 +0x7b