set to 128k.

sql/sp.cc:
  Added checking for stack overrun at functions
  db_load_routine/sp_find_routine.
sql/sp_head.cc:
  sp_head::execute() modified: pass constant value STACK_MIN_SIZE
  instead of 8 * STACK_MIN_SIZE  as second argument value
  in call to check_stack_overrun. Added checking for stack overrun
  at functions sp_lex_keeper::reset_lex_and_exec_core/sp_instr_stmt::execute.
sql/sql_parse.cc:
  check_stack_overrun modified: allocate buffer for error message
  at heap instead of stack.
  parse_sql modified: added call to check_stack_overrun() before
  parsing of sql statement.