[core] Guard against state.getItemCount() being < 0

This seems like a bug in RecyclerView but can happen if the adapter is changed multiple times between layouts.

Author: paulsowden

core/src/main/java/org/lucasr/twowayview/TwoWayLayoutManager.java

@@ -273,7 +273,7 @@ private void fillAfter(int position, Recycler recycler, State state, int extraSp
     }
 
     private void fillSpecific(int position, Recycler recycler, State state) {
-        if (state.getItemCount() == 0) {
+        if (state.getItemCount() <= 0) {
             return;
         }