fb: handle NULL pointers in framebuffer release

This function is called with a potential NULL pointer in
picolcd_init_framebuffer() and it causes a static checker warning.  This
used to handle NULL pointers when the picolcd code was written, but a
couple months later we added the "info->apertures" dereference.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>