cros: Fix login crash when determining if PIN is supported.

Bug: 847040
Change-Id: Id32b31abdfa1dff8e8229bfb4eaa610d782d4789
Reviewed-on: https://chromium-review.googlesource.com/1077017
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562611}

chrome/browser/chromeos/login/quick_unlock/pin_backend.cc

@@ -226,7 +226,7 @@ bool PinBackend::ShouldUseCryptohome(const AccountId& account_id) {
   // the prefs backend from a previous version. If that's the case, we should
   // talk to the prefs backend instead of the cryptohome backend.
   QuickUnlockStorage* storage = GetPrefsBackend(account_id);
-  return !storage->pin_storage_prefs()->IsPinSet();
+  return !storage || !storage->pin_storage_prefs()->IsPinSet();
 }
 
 }  // namespace quick_unlock