mb/google/octopus: Add null pointer check

src/mainboard/google/octopus/mainboard.c
  Function dev_find_slot may return NULL,
  check before its usage.

Found-by: klockwork
BRANCH=None
TEST=Built & booted Yorp board.

Change-Id: I160adbe3b0a5a2b0f11fd1567513860664d4bee3
Signed-off-by: John Zhao <john.zhao@intel.com>
Reviewed-on: https://review.coreboot.org/28235
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/octopus/mainboard.c

@@ -169,6 +169,7 @@ static void wifi_device_update(void)
 		devfn = PCH_DEVFN_PCIE1;
 
 	dev = dev_find_slot(0, devfn);
+	if (dev)
 		dev->enabled = 0;
 }