Fix crash: NPE in SuggestionStripView in LatinIME

The first parameter in `GestureDetector.OnGestureListener#onScroll` might be null, which indicates an incomplete event stream or error state. https://developer.android.com/reference/android/view/GestureDetector.OnGestureListener#onScroll(android.view.MotionEvent,%20android.view.MotionEvent,%20float,%20float)

Bug: 383053077
Change-Id: I6e49ae353beafb6585184fa729041570991f240c

Author: Ziqi Chen

java/src/com/android/inputmethod/latin/suggestions/SuggestionStripView.java

@@ -343,6 +343,9 @@ boolean showMoreSuggestions() {
             new GestureDetector.SimpleOnGestureListener() {
         @Override
         public boolean onScroll(MotionEvent down, MotionEvent me, float deltaX, float deltaY) {
+            if (down == null) {
+                return false;
+            }
             final float dy = me.getY() - down.getY();
             if (deltaY > 0 && dy < 0) {
                 return showMoreSuggestions();