(COUNT(*) = 1) not working in SELECT inside prepared statement.
Note: the warning was introduced in 5.0 and 5.1, 4.1 is OK with the
original fix.

The problem was that in 5.0 and 5.1 clear() for group functions may
access hybrid_type member, and this member is initialized in
fix_fields().

So we should not call clear() from item cleanup() methods, as cleanup()
may be called for unfixed items.