ui/gtk: Check if fence_fd is equal to or greater than 0

ui/gtk: Check if fence_fd is equal to or greater than 0

'fence_fd' needs to be validated always before being referenced
And the passing condition should include '== 0' as 0 is a valid
value for the file descriptor.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20240508175403.3399895-2-dongwon.kim@intel.com>
(cherry picked from commit e4e62514e3cc2fc9dbae44af8b80f61c730beab4)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>