Bug 700559 - Fix crash in startSnap() [r=pcwalton]

There exist conditions where the position of the page can change
between the call to fling() and the running of the FlingRunnable,
since the FlingRunnable gets bounced off two threads. This can
cause the Axis to end up in state WAITING_TO_SNAP with no
overscroll. Instead of crashing, handle this case more gracefully.

mobile/android/base/ui/PanZoomController.java

@@ -516,7 +516,9 @@ public class PanZoomController
                 mSnapAnim = new EaseOutAnimation(viewportPos, viewportPos - getExcess());
                 break;
             default:
-                throw new RuntimeException("Not overscrolled at startSnap()");
+                // no overscroll to deal with, so we're done
+                mFlingState = FlingStates.STOPPED;
+                return;
             }
 
             mFlingState = FlingStates.SNAPPING;