purge_sys_t::get_page(): Avoid accessing a freed reference to pages[id]
after pages.erase(id).  This heap-use-after-free would sometimes be
caught by AddressSanitizer.

purge_sys_t::iterator::free_history_rseg(): Do not crash if undo=nullptr
(the database is corrupted).

Reviewed by: Debarun Banerjee