SCUMM: Disallow saving while a room 'prequel' is shown in v0-v3 games

These are cases where the screen is completely blank, showing text.
In such cases, no room is set, thus no room resources are available.
An example is the screen shown after the credits in the Zak FM-TOWNS
intro ("The next day..."). Thanks to segrax for finding this case.
This commit is based off pull request 522, but completely disallows
saving in such scenes, instead of adding explicit checks for them