image/gif: return an error on missing palette

A frame that tries to use the global palette when it has
not been given should result in an error, not an image
with no palette at all.

Fixes #11150.

Change-Id: If0c3a201a0ac977eee2b7a5dc68930c0c5787f40
Reviewed-on: https://go-review.googlesource.com/11064Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/gif/reader.go

@@ -173,6 +173,9 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 					return err
 				}
 			} else {
+				if d.globalColorTable == nil {
+					return errors.New("gif: no color table")
+				}
 				m.Palette = d.globalColorTable
 			}
 			if d.hasTransparentIndex && int(d.transparentIndex) < len(m.Palette) {

src/image/gif/reader_test.go

@@ -217,7 +217,7 @@ func TestNoPalette(t *testing.T) {
 
 	b.WriteString(trailerStr)
 
-	try(t, b.Bytes(), "gif: invalid pixel value")
+	try(t, b.Bytes(), "gif: no color table")
 }
 
 func TestPixelOutsidePaletteRange(t *testing.T) {