automata: fix `onepass::DFA::try_search_slots` panic when too many slots are given #3038

Job	Run time
docsrs	31s
testfull-regex	2m 33s
testfull-regex-syntax	1m 46s
testfull-regex-capi	30s
cross (powerpc64-unknown-linux-gnu)	2m 5s
msrv	1m 25s
testfull-regex-automata	7m 8s
cross (aarch64-unknown-linux-gnu)	2m 10s
cross (powerpc-unknown-linux-gnu)	2m 10s
rustfmt	8s
test (beta, ubuntu-latest, beta)	2m 41s
test (win-gnu, windows-latest, stable-x86_64-gnu)	2m 59s
test (win-msvc, windows-latest, stable)	12m 28s
test (macos, macos-latest, stable)	2m 27s
test (nightly, ubuntu-latest, nightly)	2m 44s
cross (s390x-unknown-linux-gnu)	2m 1s
miri-regex-automata	10m 49s
test (stable, ubuntu-latest, stable)	2m 41s
cross (i686-unknown-linux-gnu)	1m 52s
	1h 1m 8s

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