console/dummy: leave .con_font_get set to NULL

When this method is set, the caller expects struct console_font fields
to be properly initialized when it returns. Leave it unset otherwise
nonsensical (leaked kernel stack) values are returned to user space.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>