fb: cfb: remove unnecessary `NULL` check and `NULL` assignment

Since the `fb` pointer is always assigned to `char_fb`, there is no need
for a `NULL` check. Additionally, removed setting `fb->buf` to `NULL` in
`cfb_framebuffer_init` as it will be overwritten by subsequent operations.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>