Endless loop on filenames with invalid encoding

When launching Dia with a filename argument that is not a valid codepoint in the current encoding, it enters an endless loop:

% dia $'\374'
Filename conversion failed: �
Filename conversion failed: �
Filename conversion failed: �
Filename conversion failed: �
Filename conversion failed: �
…

(The file does not need to actually exist.)

This is especially problematic when Dia is invoked by a thumbnail service¹: The thumbnailer service (e.g. Tumbler) will usually log all output to syslog, which will very quickly fill up the disk.

Implication: Users with local system access can create an appropriately named file (e.g. touch $'\374'.dia), view the parent directory in a graphical file manager, and thus cause the thumbnailer service to fill the disk (outside of any quota restriction they may have), leading to an unusable system.

On a test system, Dia produced up to 32 MiB (1.3 million lines) of error output per second.

^{1: Dia registers Exec=dia -t png -e %o -s %s %i as a thumbnailer command in /usr/share/thumbnailers/dia.thumbnailer.}