Investigate removing s2n_blob_zeroize_free

[camshaft]

Problem:

We currently have both s2n_blob_free and s2n_blob_zeroize_free which are very subtly different.

s2n_blob_zeroize_free always zeroes the memory, even if it's not an allocated blob.

s2n-tls/utils/s2n_mem.c

Lines 300 to 303 in 1209208

	POSIX_GUARD(s2n_blob_zero(b));
	if (b->allocated) {
	POSIX_GUARD(s2n_free(b));
	}

s2n_blob_free will fail if you try and free a non-growable blob:

s2n-tls/utils/s2n_mem.c

Line 285 in 1209208

POSIX_ENSURE(s2n_blob_is_growable(b), S2N_ERR_FREE_STATIC_BLOB);

however it still zeroes the blob:

s2n-tls/utils/s2n_mem.c

Line 282 in 1209208

int zero_rc = s2n_blob_zero(b);

Solution:

It would be ideal if we only had one of these. The s2n_blob_zeroize_free is only referenced in a handful of places:

$ rg s2n_blob_zeroize_free bin crypto error pq-crypto stuffer tls
tls/s2n_tls13_secrets.c
347:    DEFER_CLEANUP(struct s2n_blob shared_secret = { 0 }, s2n_blob_zeroize_free);

tls/s2n_client_key_exchange.c
229:    DEFER_CLEANUP(struct s2n_blob shared_key = { 0 }, s2n_blob_zeroize_free);
328:    DEFER_CLEANUP(struct s2n_blob shared_key = { 0 }, s2n_blob_zeroize_free);

tls/s2n_tls13_handshake.c
97:    DEFER_CLEANUP(struct s2n_blob ecdhe_shared_secret = { 0 }, s2n_blob_zeroize_free);

tls/s2n_kem.c
228:        POSIX_GUARD(s2n_blob_zeroize_free(&kem_params->private_key));
229:        POSIX_GUARD(s2n_blob_zeroize_free(&kem_params->public_key));
230:        POSIX_GUARD(s2n_blob_zeroize_free(&kem_params->shared_secret));

Assuming all of these can be replaced with s2n_blob_free, we should just get rid of s2n_blob_zeroize_free entirely.