Ignore non-existent prepared statement in get_query_string.

Ignore non-existent prepared statement in get_query_string.

CREATE FUNCTION of SQL function uses post_parse_analyze_hook during
validation.  If the function contained EXECUTE <prepared statement>,
not only that statement may be nonexistent, but also there's no need
of the query string at that point.

Since execution of nonexistent prepared statement ends with failure
sooner or later, we can safely ignore that.

Backpatch to pg_hint_plan10 that are before the core's EOL.