This bug affected queries with IN predicates that contain parameter markers
in the value list. Such queries are executed via prepared statements.
The problem appeared only if the number of elements in the value list
was greater than the set value of the system variable
in_predicate_conversion_threshold.

The patch unconditionally prohibits conversion of an IN predicate to the
equivalent IN predicand if the value list of the IN predicate contains
parameters markers.

Approved by Oleksandr Byelkin <sanja@mariadb.com>