Fixed the handling of system variable retrieval
in prepared statements : added a cleanup method
that clears up the cache and restores the 
original scope of the variable (which is overwritten
at fix_fields()).

sql/item_func.cc:
  ug #32124: 
   - preserve the requested variable scope
   - clean up the cache and restore the variable
     scope for prepared statements.
sql/item_func.h:
  Bug #32124: preserve the requested variable scope