Fix NullPointerException in DrawableContainer.mutate()

DrawableContainerState.mDrawables is an array which may be only partially
filled, as can be seen in the constructor and the addChild() method.

DrawableContainer.mutate() wrongly assumed that the array does not contain
null references.

graphics/java/android/graphics/drawable/DrawableContainer.java

@@ -234,8 +234,10 @@ public class DrawableContainer extends Drawable implements Drawable.Callback {
     @Override
     public Drawable mutate() {
         if (!mMutated && super.mutate() == this) {
-            for (Drawable child : mDrawableContainerState.mDrawables) {
-                child.mutate();
+            final int N = mDrawableContainerState.getChildCount();
+            final Drawable[] drawables = mDrawableContainerState.getChildren();
+            for (int i = 0; i < N; i++) {
+                drawables[i].mutate();
             }
             mMutated = true;
         }