compress/flate: fix out of bounds error

Fixes #2508.

R=rsc, krasin
CC=golang-dev
https://golang.org/cl/5449115

src/pkg/compress/flate/deflate.go

@@ -319,7 +319,9 @@ Loop:
 				// For matches this long, we don't bother inserting each individual
 				// item into the table.
 				d.index += d.length
-				d.hash = (int(d.window[d.index])<<hashShift + int(d.window[d.index+1]))
+				if d.index < d.maxInsertIndex {
+					d.hash = (int(d.window[d.index])<<hashShift + int(d.window[d.index+1]))
+				}
 			}
 			if d.ti == maxFlateBlockTokens {
 				// The block includes the current character

src/pkg/compress/flate/deflate_test.go

@@ -318,3 +318,15 @@ func TestWriterDict(t *testing.T) {
 		t.Fatalf("writer wrote %q want %q", b1.Bytes(), b.Bytes())
 	}
 }
+
+// See http://code.google.com/p/go/issues/detail?id=2508
+func TestRegression2508(t *testing.T) {
+	w := NewWriter(ioutil.Discard, 1)
+	buf := make([]byte, 1024)
+	for i := 0; i < 131072; i++ {
+		if _, err := w.Write(buf); err != nil {
+			t.Fatalf("writer failed: %v", err)
+		}
+	}
+	w.Close()
+}