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 🙂