The optimizer pulls up aggregate functions which should be aggregated in
an outer select. At some point it may substitute such a function for a field
in the temporary table. The setup_copy_fields function doesn't take this
into account and may overrun the copy_field buffer.
      
Fixed by filtering out the fields referenced through the specialized
reference for aggregates (Item_aggregate_ref).
Added an assertion to make sure bugs that cause similar discrepancy 
don't go undetected.

mysql-test/r/func_group.result:
  Bug #37348: test case
mysql-test/t/func_group.test:
  Bug #37348: test case
sql/item.cc:
  Bug #37348: Added a way to distinguish Item_aggregate_ref from the other types of refs
sql/item.h:
  Bug #37348: Added a way to distinguish Item_aggregate_ref from the other types of refs
sql/sql_select.cc:
  Bug #37348: 
   - Don't consider copying field references
      seen through Item_aggregate_ref
   - check for discrepancies between the number of expected 
     fields that need copying and the actual fields copied.