innodb_flush_method=wrong_value causes crash

Problem: after a failed plugin initialization, incompletely 
initialized data remained in the plugin and handlerton data 
structures. These were used later and caused the crash.

Fix: clean-up plugin related data if initialization failed.

Note: no test case added, hand tested.


sql/handler.cc:
  Fix for bug #40757: Starting server on Windows with 
  innodb_flush_method=wrong_value causes crash
    - free allocated hton and set plugin->data 
      (pointing to handlerton) to NULL if plugin->init() fails,
      as we use it as a sign that ha_initialize_handlerton() is failed,
      which is used in ha_finalize_handlerton().
    - do the same if there's no free slot for a plugin in the
      hton2plugin[] array or there are too many storage engines.
    - call plugin->deinit() in such cases as we successfully
      called plugin->init() before.