video: au1100fb: Move a variable assignment behind a null pointer check in au1100fb_setmode()

[Markus Elfring]

> Anyway, none of that applies here, because this is just pointer math.
Which data processing do you expect to be generally supported at the discussed
source code place (according to the rules of the programming language “C”)?
https://en.cppreference.com/w/c/language/behavior

Regards,
Markus