qemu-char: fix warning 'res' may be used uninitialized

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 1403683241-20678-1-git-send-email-imammedo@redhat.com
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
