Fix the `GAWK Grammar' failure.

Fix the `GAWK Grammar' failure.
* src/LR0.c (final_state): Initialize to -1 so that we do compute
the reductions of the first state which was mistakenly confused
with the final state because precisely final_state was initialized
to 0.
* tests/sets.at (Nullable): Adjust: state 0 does have lookaheads,
now noticed by Bison.
* tests/regression.at (Rule Line Numbers): Adjust: state 0 does
have a reduction on $default.