image/tiff: Do not panic when RowsPerStrip is missing.

The RowsPerStrip tag is mandatory according to the spec. However,
Mac OS sometimes (?) omits it. I managed to create such an image
by applying "tiffutil -none" on a compressed image.

If RowsPerStrip is 0, there was a division by zero in the decoder.
Assume that the image is a single strip in this case.

R=nigeltao, bradfitz
CC=golang-dev
https://golang.org/cl/4815061

src/pkg/image/tiff/reader.go

@@ -362,6 +362,10 @@ func Decode(r io.Reader) (img image.Image, err os.Error) {
 
 	// Check if we have the right number of strips, offsets and counts.
 	rps := int(d.firstVal(tRowsPerStrip))
+	if rps == 0 {
+		// Assume only one strip.
+		rps = d.config.Height
+	}
 	numStrips := (d.config.Height + rps - 1) / rps
 	if rps == 0 || len(d.features[tStripOffsets]) < numStrips || len(d.features[tStripByteCounts]) < numStrips {
 		return nil, FormatError("inconsistent header")

src/pkg/image/tiff/reader_test.go

@@ -0,0 +1,25 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tiff
+
+import (
+	"os"
+	"testing"
+)
+
+// TestNoRPS tries to decode an image that has no RowsPerStrip tag.
+// The tag is mandatory according to the spec but some software omits
+// it in the case of a single strip.
+func TestNoRPS(t *testing.T) {
+	f, err := os.Open("testdata/no_rps.tiff")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	_, err = Decode(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+}