This bug affected only multi-table update statements and in very rare
cases: one of the tables used at the top level of the statement must be
a derived table containg a row construct with a subquery including hanging
CTE.

Before this patch was applied the function prepare_unreferenced() of the
class With_element when invoked for the the hangin CTE did not properly
restored the value of thd->lex->context_analysis_only. As a result it
became 0 after the call of this function.
For a query affected by the bug this function is called when
JOIN::prepare() is called for the subquery with a hanging CTE. This happens
when Item_row::fix_fields() calls fix_fields() for the subquery. Setting
the value of thd->lex->context_analysis_only forces the caller function
Item_row::fix_fields() to invoke the virtual method is_null() for the
subquery that leads to execution of it. It causes an assertion failure
because the call of Item_row::fix_fields() happens during the invocation
of Multiupdate_prelocking_strategy::handle_end() that calls the function
mysql_derived_prepare() for the derived table used by the UPDATE at the
time when proper locks for the statement tables has not been acquired yet.

With this patch the value of thd->lex->context_analysis_only is restored
to CONTEXT_ANALYSIS_ONLY_DERIVED that is set in the function
mysql_multi_update_prepare().

Approved by Oleksandr Byelkin <sanja@mariadb.com>