splash: Prevent splash_load_fs from writing to 0x0

Passing NULL to fs_read() for actread value results in hanging U-Boot
at least on our ARM plattform (TI AM335x). Since fs_read() and
following functions do not catch nullpointers, writing to 0x0 occurs.

Passing a local dummy var instead of NULL solves this issue.

Signed-off-by: Jonathan Golder <jonathan.golder@kurz-elektronik.de>
Cc: Anatolij Gustschin <agust@denx.de>