[bugfix] Ensure that Lookup#keys is null'ed so that memory is not leaked if the query is cached for reuse

Closes eXist-db/exist#5375

Author: adamretter

exist-core/src/main/java/org/exist/xquery/Lookup.java

@@ -200,6 +200,7 @@ public void resetState(final boolean postOptimization) {
         }
 
         if (keyExpression != null) {
+            keys = null;
             keyExpression.resetState(postOptimization);
         }
     }