ui/gtk: fix leaks found wtih fuzzing

It is true, that there is no problem during runtime
from the first sight, because the memory is lost just
before qemu exits. Nevertheless, this change is necessary,
because AddressSanitizer is not able to recognize this
situation and produces crash-report (which is
false-positive in fact). Lots of False-Positive warnings
are davaluing problems, found with fuzzing, and thus the
whole methodology of dynamic analysis.
This patch eliminates such False-Positive reports,
and makes every problem, found with fuzzing, more valuable.

Fixes: 060ab763 ("gtk: don't exit early in case gtk init fails")

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20230825115818.1091936-1-frolov@swemel.ru>