automata: Fix crash in dense::DFA::from_bytes validation logic

[Mrmaxmeier]

Hi,

this PR fixes a panic in the dense::DFA::from_bytes API. The validation step after loading a corrupted DFA currently panics in TransitionTable::validate due to an invalid MatchState. This PR fixes this by validating the MatchState first.

#[test]
fn regression_validation_order() {
    let mut dfa = DFA::new("abc").unwrap();
    dfa.ms = MatchStates {
        slices: vec![],
        pattern_ids: vec![],
        pattern_len: 1,
    };
    let (buf, _) = dfa.to_bytes_native_endian();
    DFA::from_bytes(&buf).unwrap_err();
}

thread 'dfa::dense::tests::regression_validation_order' (365737) panicked at regex-automata/src/dfa/dense.rs:4644:9:
index out of bounds: the len is 0 but the index is 1
stack backtrace:
   [...]
   3: regex_automata::dfa::dense::MatchStates<T>::pattern_len
             at ./src/dfa/dense.rs:4644:9
   4: regex_automata::dfa::dense::DFA<T>::match_pattern_len
             at ./src/dfa/dense.rs:3006:17
   5: <regex_automata::dfa::dense::DFA<T> as regex_automata::dfa::automaton::Automaton>::match_len
             at ./src/dfa/dense.rs:3220:14
   6: regex_automata::dfa::dense::TransitionTable<T>::validate
             at ./src/dfa/dense.rs:3629:28
   7: regex_automata::dfa::dense::DFA<&[u32]>::from_bytes
             at ./src/dfa/dense.rs:2343:16
   8: regex_automata::dfa::dense::tests::regression_validation_order
             at ./src/dfa/dense.rs:5244:9

I'm assuming that people are not using serialized automata with untrusted inputs, so I hope just opening a PR is fine 🙂