This is a backport of
Bug #46860 Crash/segfault using EXPLAIN EXTENDED on query using UNION in subquery.


mysql-test/r/explain.result:
  Remove the extended Note, since explain returns error.
  New test case.
mysql-test/r/func_gconcat.result:
  Remove the extended Note, since explain returns error.
mysql-test/r/subselect3.result:
  Remove the extended Note, since explain returns error.
mysql-test/t/explain.test:
  New test case.
sql/sql_parse.cc:
  Skip the extended description if we have an error.