Merge change 20657

* changes:
  Fix possible NPE when mutating a DrawableContainer
diff --git a/graphics/java/android/graphics/drawable/DrawableContainer.java b/graphics/java/android/graphics/drawable/DrawableContainer.java
index dc80cf5..6b50406 100644
--- a/graphics/java/android/graphics/drawable/DrawableContainer.java
+++ b/graphics/java/android/graphics/drawable/DrawableContainer.java
@@ -237,7 +237,7 @@
             final int N = mDrawableContainerState.getChildCount();
             final Drawable[] drawables = mDrawableContainerState.getChildren();
             for (int i = 0; i < N; i++) {
-                drawables[i].mutate();
+                if (drawables[i] != null) drawables[i].mutate();
             }
             mMutated = true;
         }