[misc] ensure multi host connection close call when not succeeded

Author: rusher

src/main/java/org/mariadb/jdbc/client/impl/MultiPrimaryClient.java

@@ -488,7 +488,7 @@ public void abort(Executor executor) throws SQLException {
  @Override
  public void close() throws SQLException {
  closed = true;
- currentClient.close();
+ if (currentClient != null) currentClient.close();
  }
 
  @Override