Mark CVE-2017-18009/opencv as unfixed for unstable

The checks in modules/imgcodecs/src/grfmt_hdr.cpp in 3.2.0+dfsg-6 are
still the same as before the patch in
https://github.com/opencv/opencv/commit/4ca89db22dea962690f31c1781bce5937ee91837
and the vulnerable code seems introduced prior to the version currently
in unstable.

Check: Is there are reason it would not affect yet 3.2.0+dfsg-1 onwards?