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>