curses ui: always initialize all curses_line fields

cchar_t can contain not only attr and chars fields, but also ext_color.
Initialize the whole structure to zero instead of enumerating fields.

Spotted by Coverity: CID 1399711

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20190315130932.26094-1-samuel.thibault@ens-lyon.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>