compress/gzip: allow Reset on Reader without NewReader

Fixes #8126.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/103020044

src/pkg/compress/gzip/gunzip.go

@@ -94,7 +94,11 @@ func NewReader(r io.Reader) (*Reader, error) {
 // This permits reusing a Reader rather than allocating a new one.
 func (z *Reader) Reset(r io.Reader) error {
 	z.r = makeReader(r)
-	z.digest.Reset()
+	if z.digest == nil {
+		z.digest = crc32.NewIEEE()
+	} else {
+		z.digest.Reset()
+	}
 	z.size = 0
 	z.err = nil
 	return z.readHeader(true)

src/pkg/compress/gzip/gunzip_test.go

@@ -353,3 +353,17 @@ func TestIssue6550(t *testing.T) {
 		// ok
 	}
 }
+
+func TestInitialReset(t *testing.T) {
+	var r Reader
+	if err := r.Reset(bytes.NewReader(gunzipTests[1].gzip)); err != nil {
+		t.Error(err)
+	}
+	var buf bytes.Buffer
+	if _, err := io.Copy(&buf, &r); err != nil {
+		t.Error(err)
+	}
+	if s := buf.String(); s != gunzipTests[1].raw {
+		t.Errorf("got %q want %q", s, gunzipTests[1].raw)
+	}
+}