genirq: Ensure IRQ descriptor is valid when setting-up the IRQ

In the function, setup_irq(), we don't check that the descriptor
returned from irq_to_desc() is valid before we start using it. For
example chip_bus_lock() called from setup_irq(), assumes that the
descriptor pointer is valid and doesn't check before dereferencing it.

In many other functions including setup/free_percpu_irq() we do check
that the descriptor returned is not NULL and therefore add the same test
to setup_irq() to ensure the descriptor returned is valid.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>