Remove an inappropriate assertion in validation context.

Remove an inappropriate assertion in validation context.

spec_to_dsf() should not report invalid input by failing an assertion,
because one of the contexts it's called from is validate_desc(), in
which the whole point is to present a useful error message to the user
before getting to the point where an assertion might go off at all.

Game description 5j:c1_5o4_3c,c___aba___cca_a_a_aca now fails
validation cleanly, where previously it would trigger this bug.