Problem: Grouping results by VALUES(alias for string literal) causes
the server to crash.

Item_insert_values is not constructed to handle other types of
arguments than field and reference to field. In this case, the
argument is an Item_string, and this causes
Item_insert_values::fix_fields() to crash.

Fix: Issue an error message when the argument to Item_insert_values is
not a field or a reference to a field.

This is slightly in breach with documentation, which states that
VALUES should return NULL, but the error message is only issued in
cases where the server otherwise would crash, so there is no change in
behavior for queries that already work. Future versions will restrict
syntax so that using VALUES in this way is illegal.