Update status for CVE-2017-16232/tiff

Whilst the related commit fixes a memory leak issue in the tiff2bw tool,
the underlying memory-based DOS still would be possible as described in
https://seclists.org/oss-sec/2017/q4/168 and in
https://gitlab.com/libtiff/libtiff/commit/25f9ffa56548c1846c4a1f19308b7f561f7b1ab0
.

Mark again as unfixed. The issue itself -- affecting only tiff2bw -- is
still to be considered unimportant.