compress/flate: fix infinite loop on malformed data

Test using compress/gzip, because that's how the
data arrived.

Fixes #6550.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/14441051

src/pkg/compress/flate/inflate.go

@@ -644,6 +644,10 @@ func (f *decompressor) huffSym(h *huffmanDecoder) (int, error) {
 		if n > huffmanChunkBits {
 			chunk = h.links[chunk>>huffmanValueShift][(f.b>>huffmanChunkBits)&h.linkMask]
 			n = uint(chunk & huffmanCountMask)
+			if n == 0 {
+				f.err = CorruptInputError(f.roffset)
+				return 0, f.err
+			}
 		}
 		if n <= f.nb {
 			f.b >>= n

src/pkg/compress/gzip/gunzip_test.go

@@ -7,7 +7,10 @@ package gzip
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
+	"os"
 	"testing"
+	"time"
 )
 
 type gunzipTest struct {
@@ -302,3 +305,31 @@ func TestDecompressor(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue6550(t *testing.T) {
+	f, err := os.Open("testdata/issue6550.gz")
+	if err != nil {
+		t.Fatal(err)
+	}
+	gzip, err := NewReader(f)
+	if err != nil {
+		t.Fatalf("NewReader(testdata/issue6550.gz): %v", err)
+	}
+	defer gzip.Close()
+	done := make(chan bool, 1)
+	go func() {
+		_, err := io.Copy(ioutil.Discard, gzip)
+		if err == nil {
+			t.Errorf("Copy succeeded")
+		} else {
+			t.Logf("Copy failed (correctly): %v", err)
+		}
+		done <- true
+	}()
+	select {
+	case <-time.After(1 * time.Second):
+		t.Errorf("Copy hung")
+	case <-done:
+		// ok
+	}
+}