Add null check in ViewOverlay

View.getOverlay().clear() can failed with an NPE if there are
no drawables in the overlay. Fix: add a null check before dereferencing
the mDrawables field.

Issue #8895794 getOverlay.clear() crashes if drawables were not added previously

Change-Id: I9b2a63036450915681ba3a89a0911e2490063702

core/java/android/view/ViewOverlay.java

@@ -179,7 +179,9 @@ public class ViewOverlay {
 
         public void clear() {
             removeAllViews();
-            mDrawables.clear();
+            if (mDrawables != null) {
+                mDrawables.clear();
+            }
         }
 
         boolean isEmpty() {