common/lcd: add protection from null bmp pointer

If the bmp pointer is null then U-Boot will get stuck when trying
to load the image.
What's worse, it will get stuck before the U-Boot shell becomes
available to the user, thus making it difficult to correct the
situation.

To protect from the above scenario, check if the pointer is valid.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>