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Update hkdf.c to avoid potentially vulnerable code pattern

The expression "if (a+b>c) a=c-b" is incorrect if "a+b" overflows.
It should be replaced by "if (a>c-b) a=c-b", which avoids the
potential overflow and is much easier to understand.

This pattern is the root cause of CVE-2022-37454, a buffer overflow
vulnerability in the "official" SHA-3 implementation.

It has been confirmed that the addition in
https://github.com/openssl/openssl/blob/master/providers/implementations/kdfs/hkdf.c#L534
cannot overflow. So this is only a minor change proposal to avoid
a potentially vulnerable code pattern and to improve readability.
More information: https://github.com/github/codeql/pull/12036#issuecomment-1466056959

CLA: trivial

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20990)

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