automata: remove some unsafe code #2239

Job	Run time
testfull-regex	3m 50s
msrv	2m 8s
rustfmt	13s
miri-regex-automata	10m 47s
test (stable, ubuntu-latest, stable)	4m 2s
testfull-regex-automata	8m 30s
testfull-regex-capi	29s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	3m 7s
testfull-regex-syntax	2m 10s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 59s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	10m 18s
test (beta, ubuntu-latest, beta)	4m 9s
test (nightly, ubuntu-latest, nightly)	4m 2s
test (macos, macos-latest, stable)	3m 3s
test (win-msvc, windows-latest, stable)	6m 35s
test (win-gnu, windows-latest, stable-x86_64-gnu)	4m 1s
	1h 15m 23s

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