expression cols.

The problem was that MYSQL_FIELD::org_name was set for MIN() and MAX()
functions (COUNT() is also mentioned in the bug report but was already
fixed).

After this patch for expressions MYSQL_FIELD::name is set to either
expression itself or its alias, and other data origin fields of
MYSQL_FILED (db, org_table, table, org_name) are empty strings.


sql/item_sum.cc:
  For expressions only col_name should be non-empty string.
tests/mysql_client_test.c:
  Add test case for bug#21635: MYSQL_FIELD struct's member strings seem
  to misbehave for expression cols.