[PATCH] x86: irq: Check for valid irq descriptor in check_irq_vectors_for_cpu_disable

[Joerg Roedel]

Hi,

here is a patch to fix a kernel panic at shutdown we have seen recently.
The issue is hard to reproduce, so the patch description about the root
cause of the bug comes only from review and my current understanding of
x86 irq code.

So if what I wrote in the patch description doesn't make sense, please
let me know.

Constructive comments and feedback welcome.

Thanks,

	Joerg