dfa: attempt to pacify Coverity

dfa: attempt to pacify Coverity

* lib/dfa.c (lex): Use ‘assume’ rather than ‘abort’,
to try to pacify Coverity.
(maybe_disable_superset_dfa): Use ‘assume’ here too, for consistency.
Using ‘assume’ should make the code a tiny bit faster, though
at the cost of having undefined behavior instead of nicely aborting.