The problem arose because the automatic key generation part of
derived table handling did not handle properly columns specified as
part of the VALUES() clause.

As a side note, the VALUES() clause (that is not used as a
<table value constructor>) is seriously flawed and should be replaced
with some other mechanism.

What happens is that when VALUES() is detected outside where it is
meaningful, its value should be NULL and is thus replaced with a
Field_null object, with the same name as the original field.
In update_derived_keys() this is let through, and when a key is added in
TABLE::add_tmp_key(), the generated field's name is compared with the
table`s field names, which contains a blob field with the same name.
This field type is not accepted and an assert is triggered.

The solution appears to be to replace the NULL field with a NULL const
instead. Fields with type NULL are generally not handled well inside the
server (e.g Innnodb will not accept them and it is impossible to create
them in regular tables), so this seems like an overall good change.