Fix TextView potential NPE in isLayoutRtl()

- test if mDrawables can be null
- see bug #4517017 (was seen with Monkeys)

Change-Id: I07f04c88a238d1cae6fe89b0ab56fc2c456b8534
diff --git a/core/java/android/widget/TextView.java b/core/java/android/widget/TextView.java
index a73a6cf..34f748a 100644
--- a/core/java/android/widget/TextView.java
+++ b/core/java/android/widget/TextView.java
@@ -4144,10 +4144,12 @@
     @Override
     public boolean isLayoutRtl(Drawable who) {
         if (who == null) return false;
-        final TextView.Drawables drawables = mDrawables;
-        if (who == drawables.mDrawableLeft || who == drawables.mDrawableRight ||
-            who == drawables.mDrawableTop || who == drawables.mDrawableBottom) {
-            return isLayoutRtl();
+        if (mDrawables != null) {
+            final Drawables drawables = mDrawables;
+            if (who == drawables.mDrawableLeft || who == drawables.mDrawableRight ||
+                who == drawables.mDrawableTop || who == drawables.mDrawableBottom) {
+                return isLayoutRtl();
+            }
         }
         return super.isLayoutRtl(who);
     }