Fix NullPointerException in GestureDetector

NPE can be thrown from GestureDetector on GestureDetector#cancel which
can crash an app as VelocityTracker object is not checked for null
before calling VelocityTracker#recycle.
This change fixes the issue by checking for null VelocityTracker before
calling VelocityTracker#recycle on GestureDetector#cancel.

Test: Precautionary fix to prevent crashes, no test was run

Change-Id: I805ac4a19fbd766a9d0f17732b158f1d1e61aae6

core/java/android/view/GestureDetector.java

@@ -839,8 +839,10 @@ public class GestureDetector {
         mHandler.removeMessages(SHOW_PRESS);
         mHandler.removeMessages(LONG_PRESS);
         mHandler.removeMessages(TAP);
-        mVelocityTracker.recycle();
-        mVelocityTracker = null;
+        if (mVelocityTracker != null) {
+            mVelocityTracker.recycle();
+            mVelocityTracker = null;
+        }
         mIsDoubleTapping = false;
         mStillDown = false;
         mAlwaysInTapRegion = false;