cmd/internal/buildid: add missing f.Close in ReadFile

On Windows, not closing f keeps us from being able to remove it.

Change-Id: Id4cb709b6ce0b30485b87364a9f0e6e71d2782bd
Reviewed-on: https://go-review.googlesource.com/70070
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/buildid/buildid.go

@@ -33,6 +33,7 @@ func ReadFile(name string) (id string, err error) {
 	if err != nil {
 		return "", err
 	}
+	defer f.Close()
 
 	buf := make([]byte, 8)
 	if _, err := f.ReadAt(buf, 0); err != nil {
@@ -54,8 +55,6 @@ func ReadFile(name string) (id string, err error) {
 	// Reading the first 1024 bytes should be plenty.
 	data := make([]byte, 1024)
 	n, err := io.ReadFull(f, data)
-	f.Close()
-
 	if err != nil && n == 0 {
 		return "", err
 	}