MDEV-22910: SIGSEGV in Opt_trace_context::is_started & SIGSEGV in Json_writer::add_table_name (on optimized builds)

Make sure to initialize members of TABLE::reginfo when TABLE::init is called. In this case the problem
was that table->reginfo.join_tab was set for the SELECT query and then was reused by the UPDATE query.
This case occurred only when the SELECT query had a degenerate join.