Fix crash on filename conversion error

* src/fileman.c (add_file): Handle conversion errors.

If g_filename_to_utf8 fails we now fall back to g_locale_to_utf8.
If this still does not work we fall back to g_filename_display_name
which replaces unconvertibale strings by question marks or unicode
markup.
Previously NULL pointer would be inserted as filenames, leading
to crashes later on.

This is especially important for windows where D&D files came
in System encoding as well as "Double clicked" or "Open With" files.
On windows filename_to_utf8 always assumes that the input is already
UTF-8, because it's stupid. (or because the GTK File Dialog returns
UTF-8 filenames) so the fallback to locale is especially important
here.

• GnuPG-Bug-ID: T2185