cmd/pprof: fix crash with invalid source

pprof crashed when running with:

     go tool pprof %INVALID

Change-Id: I47c2a4da7273e3a97a833f320a650b8c6b59d6b9
Reviewed-on: https://go-review.googlesource.com/20632Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/pprof/internal/driver/driver.go

@@ -139,7 +139,7 @@ func adjustURL(source string, sec int, ui plugin.UI) (adjusted, host string, dur
 	if err != nil || (url.Host == "" && url.Scheme != "" && url.Scheme != "file") {
 		url, err = url.Parse("http://" + source)
 		if err != nil {
-			return source, url.Host, time.Duration(30) * time.Second
+			return source, "", 0
 		}
 	}
 	if scheme := strings.ToLower(url.Scheme); scheme == "" || scheme == "file" {