refactor: make silk_decode_core safe #60

Job	Run time
test-vectors (ubuntu-latest)	2m 15s
test-basic (ubuntu-latest, release)	1m 55s
test-vectors (macos-13)	2m 16s
test-basic (ubuntu-latest, dev)	13m 32s
test-vectors (macos-14)	1m 16s
test-basic (macos-13, release)	2m 14s
test-vectors (windows-latest)	4m 27s
test-basic (macos-13, dev)	14m 11s
test-basic (macos-14, release)	1m 25s
test-basic (macos-14, dev)	11m 0s
test-basic (windows-latest, release)	4m 33s
test-basic (windows-latest, dev)	17m 47s
	1h 16m 51s

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