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 <[email protected]>

 riff/riff.go   | 7 +++----
 webp/decode.go | 3 +++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/riff/riff.go b/riff/riff.go
index b225193..9b9f71d 100644
--- a/riff/riff.go
+++ b/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 {
diff --git a/webp/decode.go b/webp/decode.go
index 4a91f23..259bac6 100644
--- a/webp/decode.go
+++ b/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
 		}
 	}
 }