produce wrong data

By default Item_sp_func::val_str() returns string from it's result_field 
internal buffer. When grouping is present Item_copy_string is used to 
store SP function result, but it doesn't additionally buffer the result.
When the next record is read, internal buffer is overwritten, due to
this Item_copy_string::val_str() will have wrong data. Thus producing
weird query result.

The Item_func_sp::val_str() now makes a copy of returned value to prevent
occasional corruption.


mysql-test/t/sp.test:
  Added test case for bug#13575: SP funcs in select with distinct/group and order by can
  produce wrong data
mysql-test/r/sp.result:
  Added test case for bug#13575: SP funcs in select with distinct/group and
      order by can produce wrong data
sql/item_func.h:
  Fixed bug#13575: SP funcs in select with distinct/group and order by can
      produce wrong data
      The Item_func_sp::val_str() now makes a copy of returned value to prevent
      occasinal corruption.