Fix return value from newgame_undo_deserialise_read.

Fix return value from newgame_undo_deserialise_read.

The read function used by midend_deserialise and friends is expected
never to perform a partial read (the main deserialisation code always
knows how many bytes it can expect to see), so it's specified to
return simply TRUE or FALSE for success/failure, rather than the
number of bytes read.

This probably wasn't breaking anything, since in the case of
deserialising from an internal memory buffer a short read could only
arise due to an outright bug constructing the buffer. But now I've
spotted it, I should fix it.