image/gif: fix nil deref in test

LGTM=crawshaw, dave
R=golang-codereviews, crawshaw, dave
CC=golang-codereviews
https://golang.org/cl/104520044

src/pkg/image/gif/writer_test.go

@@ -116,7 +116,7 @@ func TestEncodeAll(t *testing.T) {
 	for i, f := range frames {
 		m, err := readGIF(f)
 		if err != nil {
-			t.Error(f, err)
+			t.Fatal(f, err)
 		}
 		g0.Image[i] = m.Image[0]
 	}