Capture RowStreamImpl endHandler in a synchronized block

Follows-up on eclipse-vertx#1577

The endHandler was no longer captured safely after changes in the previous PR.

Signed-off-by: Thomas Segismont <tsegismont@gmail.com>

Author: tsegismont

vertx-sql-client/src/main/java/io/vertx/sqlclient/impl/RowStreamImpl.java

@@ -203,8 +203,9 @@ private void checkPending() {
               break;
             } else {
               cursor.close();
+              final Handler<Void> eh = endHandler;
               handler = v -> {
-                endHandler.handle(null);
+                eh.handle(null);
                 synchronized (RowStreamImpl.this) {
                   cursor = null;
                 }