KEY HANDLING ON SUBSEQUENT CREATE TABLE IF NOT EXISTS
      
      PROBLEM:
      --------
      Consider a SP routine which does CREATE TABLE
      with REFERENCES clause. The first call to this routine
      invokes parser and the parsed items are cached, so as 
      to avoid parsing for the second execution of the routine.
      
      It is obsevered that valgrind reports a warning
      upon read of thd->lex->alter_info->key_list->Foreign_key object,
      which seem to be pointing to a invalid memory address
      during second time execution of the routine. Accessing this object
      theoretically could cause a crash.
      
      ANALYSIS:
      ---------
      The problem stems from the fact that for some reason
      elements of ref_columns list in thd->lex->alter_info->
      key_list->Foreign_key object are changed to point to
      objects allocated on runtime memory root.
      
      During the first execution of routine we create
      a copy of thd->lex->alter_info object.
      As part of this process we create a clones of objects in
      Alter_info::key_list and of Foreign_key object in particular.
      Then Foreign_key object is cloned for some reason we
      perform shallow copies of both Foreign_key::ref_columns
      and Foreign_key::columns list. So new instance of 
      Foreign_key object starts to SHARE contents of ref_columns
      and columns list with the original instance.
      After that as part of cloning process we call
      list_copy_and_replace_each_value() for elements of
      ref_columns list. As result ref_columns lists in both
      original and cloned Foreign_key object start to contain
      pointers to Key_part_spec objects allocated on runtime
      memory root because of shallow copy.
      
      So when we start copying of thd->lex->alter_info object
      during the second execution of stored routine we indeed
      encounter pointer to the Key_part_spec object allocated
      on runtime mem-root which was cleared during at the end
      of previous execution. This is done in sp_head::execute(), 
      by a call to free_root(&execute_mem_root,MYF(0));
      As result we get valgrind warnings about accessing 
      unreferenced memory.
      
      FIX:
      ----
      The safest solution to this problem is to 
      fix Foreign_key(Foreign_key, MEM_ROOT) constructor to do
      a deep copy of columns lists, similar to Key(Key, MEM_ROOT) 
      constructor.