NULL function pointers are very bad security wise. This one got caught by
kerneloops.org quite a few times, so it's happening in the field....

Fix is simple, check the function pointer for NULL, like 6 other places
in the same function are already doing.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>