in load_defaults() 

load_defaults(), my_search_option_files() and 
my_print_default_files()  utilized a global variable 
containing  a pointer to thread local memory. This could lead 
to race conditions when those functions were called with high 
concurrency. 

Fixed by changing the interface of the said functions to avoid 
the necessity for using a global variable.

Since we cannot change load_defaults() prototype for API
compatibility reasons, it was renamed my_load_defaults().
Now load_defaults() is a thread-unsafe wrapper around
a thread-safe version, my_load_defaults().


mysys/default.c:
  1. Added a thread-safe version of load_defaults(), changed
  load_defaults() with the old interface to be a thread-unsafe
  wrapper around the thread-safe version.
  2. Always use a private MEM_ROOT in my_print_default_files, 
  don't use a global variable.
sql-common/client.c:
  Use a thread-safe version of load_defaults().