correct sql_alloc, ORDER BY and NULL value in group_concat
add comments
move test msg_arg
add test on NULL


mysql-test/r/func_gconcat.result:
  change work with NULL
mysql-test/t/func_gconcat.test:
  add test on NULL
sql/item_sum.cc:
  correct sql_alloc, ORDER BY and NULL value
  add comments
sql/sql_class.h:
  move test msg_arg