fix_fields() was not called for "order by" variables if the type was a
  "constant integer", and thus interpreted as a column index.
  However, a local variable is an expression and should not be interpreted
  as a column index. Instead it behaves just like when using a user variable
  for instance (i.e. it will not affect the ordering).



mysql-test/r/sp.result:
  Updated results for new test case (BUG#16474).
mysql-test/t/sp.test:
  New test case for BUG#16474.
sql/sql_select.cc:
  When processing order list,