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: e2e64a932556 ("genirq: Set initial affinity in irq_set_affinity_hint()")
Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: netdev@vger.kernel.org
Link: http://lkml.kernel.org/r/20150128185739.9689.84588.stgit@jbrandeb-cp2.jf.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>
Change-Id: I7688a865826e0985947982b65cd71fd4b04fde48
Git-repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Git-commit: 4fe7ffb7e17ca6ad9173b8de35f260c9c8fc2f79
Signed-off-by: Zhiqiang Tu <ztu@codeaurora.org>