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>
diff --git a/src/mainboard/google/octopus/mainboard.c b/src/mainboard/google/octopus/mainboard.c
index 61d254e..6effa7c 100644
--- a/src/mainboard/google/octopus/mainboard.c
+++ b/src/mainboard/google/octopus/mainboard.c
@@ -169,7 +169,8 @@
 		devfn = PCH_DEVFN_PCIE1;
 
 	dev = dev_find_slot(0, devfn);
-	dev->enabled = 0;
+	if (dev)
+		dev->enabled = 0;
 }
 
 void mainboard_devtree_update(struct device *dev)