Add a test that excercises the 'no conversion' code path.
This uncovered that we don't treat errno properly in this path,
and as a consequence, the returned error code is unreliable.
These don't really matter, since it's test code, but they do obscure
real leaks in the library.
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=666115
Acked-by: Matthias Clasen <mclasen@redhat.com>