genirq: Fix null pointer reference in irq_set_affinity_hint()

The recent set_affinity commit by me introduced some null
pointer dereferences on driver unload, because some drivers
call this function with a NULL argument. This fixes the issue
by just checking for null before setting the affinity mask.

Fixes: e2e64a93 ("genirq: Set initial affinity in irq_set_affinity_hint()")
Reported-by: NYinghai Lu <yinghai@kernel.org>
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
CC: netdev@vger.kernel.org
Link: http://lkml.kernel.org/r/20150128185739.9689.84588.stgit@jbrandeb-cp2.jf.intel.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/irq/manage.c

@@ -244,6 +244,7 @@ int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
 	desc->affinity_hint = m;
 	irq_put_desc_unlock(desc, flags);
 	/* set the initial affinity to prevent every interrupt being on CPU0 */
+	if (m)
 		__irq_set_affinity(irq, m, false);
 	return 0;
 }