Fix for MDEV-31466 - add optional derived table column names.
Column names within a SELECT_LEX structure can be left in a non-reparsable
state (as printed out from *::print) after JOIN::prepare.  This caused
an incorrect view definition to be written into the .FRM file.
Fixed by resetting item list names in SELECT_LEX structures representing
derived tables before writing out the view definition.

Reviewed by Igor Babaev (igor@mariadb.com)