Fix calls to BNFreeString in kernel cache API

[spoonmilk]

Amends calls to BNFreeString in kernel cache API that were causing get_macho_header_for_address, get_macho_header_for_image, get_image_name_for_address, and get_name_for_address to fail.

Fixes #6855

[CLAassistant]

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3 out of 4 committers have signed the CLA.

✅ spoonmilk
✅ bdash
✅ mostobriv
❌ D0ntPanic
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[emesare]

LGTM

[fuzyll]

This branch got a little weird somehow, so I just cherry-picked the commit onto dev. Thanks!