map2preview: handle unknown tilesets

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Prevent out-of-bounds array access

GCC actually gives a warning for this.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Prevent updating autorevision.h's timestamp when its contents haven't changed

Signed-off-by: Giel van Schijndel <me@mortis.eu>