vp8l: don't ignore I/O errors when reading LZ77 parameters.

Fixes golang/go#10383

Change-Id: I3e5e357a40a45189c31f0b4db9422dedc351c910
Reviewed-on: https://go-review.googlesource.com/9912
Reviewed-by: Nigel Tao <nigeltao@golang.org>

vp8l/decode.go

@@ -496,7 +496,7 @@ func (d *decoder) lz77Param(symbol uint32) (uint32, error) {
 	offset := (2 + symbol&1) << extraBits
 	n, err := d.read(extraBits)
 	if err != nil {
-		return 0, nil
+		return 0, err
 	}
 	return offset + n + 1, nil
 }