apacheGH-653: Nullify fieldReader when invalidating parent object (apache#654)

## What's Changed

Set the fieldReader object to null when invalidating the parent reader.
This will prevent odd behavior if the object is reused.


Closes apache#653

Author: lriggs

vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -769,6 +769,7 @@ public UnionVector promoteToUnion() {
   }
 
   protected void invalidateReader() {
+    fieldReader = null;
     reader = null;
   }