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>
2 files changed