tiff/lzw: sync (again) to the upstream lzw in the stdlib.
tiff/lzw: sync (again) to the upstream lzw in the stdlib.

I somehow missed this in the previous change. Not sure how.

Change-Id: Ib9277944418a73cda6acc07fc308d846b4064b9e
Reviewed-on: https://go-review.googlesource.com/11321
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

golang/image