genirq: Check __free_irq() return value for NULL

__free_irq() can return a NULL irqaction for example when trying to free
already-free IRQ, but the callsite unconditionally dereferences the
returned pointer.

Fix this by adding a check and return NULL.

Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20170919200412.GA29985@gmail.com