Fix off-by-one in LZW decoding

See http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-July/004566.html.

The file listed in the thread should now open as it does with imread/libtiff.  I would expect several jobs to fail though due to a handful of filesets having changed pixels hashes.  It would probably be a good idea to verify anything that fails with this PR against imread or libtiff as well, just to make sure this doesn't break images that were correct before.