IsKeyboardPresentOnSlate(): null check before assigning to result

Bug: 805881
Change-Id: I29434a3e34a816412696188674ae9ecf47e6fed6
Reviewed-on: https://chromium-review.googlesource.com/899945Reviewed-by: Justin Schuh <jschuh@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534204}

base/win/win_util.cc

@@ -166,7 +166,8 @@ bool IsKeyboardPresentOnSlate(std::string* reason, HWND hwnd) {
   bool result = false;
 
   if (GetVersion() < VERSION_WIN8) {
-    *reason = "Detection not supported";
+    if (reason)
+      *reason = "Detection not supported";
     return false;
   }