fixed potential crash bug for special versions without "classic" artwork

This bug can happen when using the program binary within a special,
customized version of R'n'D like "R'n'D jue" that does not contain
the usual "classic" artwork (like the graphics set "gfx_classic").

The problem is solved by doing a fallback to the default artwork set
(as it can be defined using "default_graphics_set") if the "classic"
artwork set cannot be found.