Fix usage of bytes.NewBuffer in bpfix

According to the documentation: "NewBuffer creates and initializes a new
Buffer using buf as its initial contents. The new Buffer takes ownership
of buf, and the caller should not use buf after this call."

Test: Run bpfix twice, only wrote the first time.
Change-Id: I52f88bfd9247240436b46f396c9196157774615b

bpfix/cmd/bpfix.go

@@ -65,7 +65,7 @@ func processFile(filename string, in io.Reader, out io.Writer, fixRequest bpfix.
 	if err != nil {
 		return err
 	}
-	r := bytes.NewBuffer(src)
+	r := bytes.NewBuffer(append([]byte(nil), src...))
 	file, errs := parser.Parse(filename, r, parser.NewScope(nil))
 	if len(errs) > 0 {
 		for _, err := range errs {