video: Check for valid FB pointer before clearing

This command will start erasing at memory address zero
if there is not a valid framebuffer address that was found
during video_init().

This is a common case with Chrome OS devices in normal mode
when we do not execute the video option rom in coreboot.

Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>