caused a few tests to fail because the thd->extra_lock wasn't being set to
NULL after the table was unlocked. This poses a serious problem because later
attempts to access thd->extra_lock (now a dangling pointer) will probably
result in a crash (undefined behavior) -- and that's what actually happens
in some test cases.

The solution is to set the select_create::m_plock pointee to NULL, which
means that thd->extra_lock is set to NULL when the lock data is not for a
temporary table.