char: applicom: fix NULL pointer dereference in ac_ioctl

In the Linux kernel, the following vulnerability has been resolved:

char: applicom: fix NULL pointer dereference in ac_ioctl

Discovered by Atuin - Automated Vulnerability Discovery Engine.

In ac_ioctl, the validation of IndexCard and the check for a valid

RamIO pointer are skipped when cmd is 6. However, the function

unconditionally executes readb(apbs[IndexCard].RamIO + VERS) at the

end.

If cmd is 6, IndexCard may reference a board that does not exist

(where RamIO is NULL), leading to a NULL pointer dereference.

Fix this by skipping the readb access when cmd is 6, as this

command is a global information query and does not target a specific

board context.