webp: fix panic in case of invalid chunkID

At the time of decoding webp file, if chunkID is not one of
{'ALPH', 'VP8', 'VP8L', 'VP8X'} return errInvalidFormat

Fixes golang/go#10384

Change-Id: I167909b5ddef174d161f806297a08fac6aabcf19
Reviewed-on: https://go-review.googlesource.com/9839
Reviewed-by: Nigel Tao <nigeltao@golang.org>

riff/riff.go

@@ -136,11 +136,10 @@ func (z *Reader) Next() (chunkID FourCC, chunkLen uint32, chunkData io.Reader, e
 		return FourCC{}, 0, nil, z.err
 	}
 	chunkID = FourCC{z.buf[0], z.buf[1], z.buf[2], z.buf[3]}
-	chunkLen = u32(z.buf[4:])
-	z.chunkLen = chunkLen
-	z.padded = chunkLen&1 == 1
+	z.chunkLen = u32(z.buf[4:])
+	z.padded = z.chunkLen&1 == 1
 	z.chunkReader = &chunkReader{z}
-	return chunkID, chunkLen, z.chunkReader, nil
+	return chunkID, z.chunkLen, z.chunkReader, nil
 }
 
 type chunkReader struct {

webp/decode.go

@@ -144,6 +144,9 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
 				}, nil
 			}
 			wantAlpha = true
+
+		default:
+			return nil, image.Config{}, errInvalidFormat
 		}
 	}
 }