my_thread_global_init() + my_thrad_global_end() repeatadily.
This caused THR_KEY_mysys to be allocated multiple times.

Deletion of THR_KEY_mysys was originally in my_thread_global_end() but was
moved to my_end() as DBUG uses THR_KEY_mysys and DBUG is released after
my_thread_global_end() is called.

Releasing DBUG before my_thread_global_end() and move THR_KEY_mysys back
into my_thread_global_end() could be a solution, but as safe_mutex and other
things called by my_thread_global_end is using DBUG it may not be completely
safe.

To solve this, I used the simple solution to add a marker that THR_KEY_mysys
is created and not re-create it in my_thread_global_init if it already
exists.