video:uvesafb: check the return value of kzalloc

Michal maybe forgot it merely, we should add code
to check the return value of kzalloc to make the
code more robust.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>