Fix the crash using DiffUtil with FlexboxLayoutManager. (#425)

Fixes #332

Author: thagikura

flexbox/src/main/java/com/google/android/flexbox/FlexboxLayoutManager.java

@@ -2876,7 +2876,8 @@ private void assignFromView(View anchor) {
             mPosition = getPosition(anchor);
             mAssignedFromSavedState = false;
             assert mFlexboxHelper.mIndexToFlexLine != null;
-            int flexLinePosition = mFlexboxHelper.mIndexToFlexLine[mPosition];
+            int flexLinePosition =
+                    mFlexboxHelper.mIndexToFlexLine[mPosition != NO_POSITION ? mPosition : 0];
             mFlexLinePosition = flexLinePosition != NO_POSITION ? flexLinePosition : 0;
             // It's likely that the view is the first item in a flex line, but if not get the
             // index of the first item in the same line because the calculation of the flex lines