Avoid crashing when Statement#close is called before a Result is collected

[sodabrew]

Alternative to #692 / Resolves #690

I found that you can call mysql_stmt_close first, without calling mysql_stmt_result_free, however you must not later call mysql_stmt_result_free.

[sodabrew]

@kamipo What do you think of doing it this way, rather than providing an explicit Result#free that the user must call?

[kamipo]

I think it is better to call immediately mysql_stmt_free_result to a result set that produced by mysql_stmt_store_result. But actually this is Rubyish way, it looks good to me.
I PRed rails/rails#22352 working on this branch. It looks like works fine. Thanks!

[sodabrew]

@kamipo I asked on that Rails PR, and should probably ask here for continuity, if you require the Mysql2::Result#free method in order to hook this into the ActiveRecord adapter, or if allowing Ruby GC to handle the result free is sufficient?

[kamipo]

I realized that when if (!is_streaming), Mysql2::Result#each is called in Statement#execute, then rb_mysql_result_free_result is called immediately. It seems good to me. Mysql2::Result#free is not needed for now.

https://github.com/sodabrew/mysql2/blob/ed5613d26b1f54e5eb0629a06cb5136de4ae47cf/ext/mysql2/statement.c#L374-L377

  if (!is_streaming) {
    // cache all result
    rb_funcall(resultObj, intern_each, 0);
  }

https://github.com/sodabrew/mysql2/blob/ed5613d26b1f54e5eb0629a06cb5136de4ae47cf/ext/mysql2/result.c#L840-L843

      if (wrapper->lastRowProcessed == wrapper->numberOfRows) {
        /* we don't need the mysql C dataset around anymore, peace it */
        rb_mysql_result_free_result(wrapper);
      }