ui: avoid compiler warnings from unused clipboard info variable

ui: avoid compiler warnings from unused clipboard info variable

With latest clang 13.0.0 we get

../ui/clipboard.c:47:34: error: variable 'old' set but not used [-Werror,-Wunused-but-set-variable]
    g_autoptr(QemuClipboardInfo) old = NULL;
                                 ^

The compiler can't tell that we only declared this variable in
order to get the side effect of free'ing it when out of scope.

This pattern is a little dubious for a use of g_autoptr, so
rewrite the code to avoid it.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
[AJB: fix merge conflict]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211215141949.3512719-2-berrange@redhat.com>
Message-Id: <20220105135009.1584676-2-alex.bennee@linaro.org>