Fixes abort when the old specifiers are used. Fix up the conversion
specifier, it would get overwritten with 'c' below to the extension
format char, which then later is unhandled, leading to the abort.
Also fix up and enable unit test for this.
https://bugzilla.gnome.org/process_bug.cgi
We will add support for our own printf modifiers, so we can
get nice debug log output on all operating systems irrespective
of the specific libc version used.
https://bugzilla.gnome.org/show_bug.cgi?id=613081