KRAU8Y4HWH - Fix Notes client crash when Xpage opens and saves a certain type of external emails...

SPR# KRAU8Y4HWH - Fix Notes client crash when Xpage opens and saves a certain type of external emails with MIME. The problem is that the client crashes due to a stale block ID of the item in MIME Header when the backend classes call a special CMimeDirectory API, OpenForEdit(), to open a Directory for editing. This is a defensive fix to validate the block ID of the item in the Mime Header if the backend classes call for OpenForEdit..