x86/irq: Do not dereference irq descriptor before checking it

Having the IS_NULL_OR_ERR() check after dereferencing the pointer is
not really working well.

Move the dereference after the check.

Fixes: a782a7e4 'x86/irq: Store irq descriptor in vector array'
Reported-and-tested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>