LZW: fix one more off-by-one

See http://www.openmicroscopy.org/community/viewtopic.php?f=13&t=7705.  For each of the three files linked in the forum thread, the values of the last column of pixels should be compared against the last column of pixels returned by ```imread``` in Matlab.  Without this change, each of the files should have a handful of rows for which the values differ; with this change, the values should match for all rows.

See also gh-1200.