Fix crash setting null seekbar thumb

External bug http://code.google.com/p/android/issues/detail?id=15693

Change-Id: I2f869b4ebd1f0857564c69999abb49285beefd14

Author: adamp

core/java/android/widget/AbsSeekBar.java

@@ -123,7 +123,7 @@ public void setThumb(Drawable thumb) {
         invalidate();
         if (needUpdate) {
             updateThumbPos(getWidth(), getHeight());
-            if (thumb.isStateful()) {
+            if (thumb != null && thumb.isStateful()) {
                 // Note that if the states are different this won't work.
                 // For now, let's consider that an app bug.
                 int[] state = getDrawableState();