prevent NPE in divider decoration

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/ui/util/recyclerview/DividerItemDecoration.java

@@ -102,6 +102,11 @@ public class DividerItemDecoration extends RecyclerView.ItemDecoration {
     @Override
     public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
                                RecyclerView.State state) {
+        if (mDivider == null) {
+            outRect.set(0, 0, 0, 0);
+            return;
+        }
+
         if (mOrientation == VERTICAL_LIST) {
             outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
         } else {