providers: Nullify BIO pointer after free to prevent double free

providers: Nullify BIO pointer after free to prevent double free

In providers/implementations/storemgmt/file_store_any2obj.c, if the
control flow reaches the err label after BIO_free(in) is called, a
double free will occur in the generic cleanup block.

Currently, the only path to this specific err jump is if
BUF_MEM_grow(mem, len) fails. As noted by the OpenSSL Security Team,
this failure is currently impossible because the buffer is being
shrunk (max_len >= len).

However, as requested by the security team via email, this commit
explicitly nullifies the in pointer after the first free to
future-proof the function and prevent a double free in case the
semantics of BUF_MEM_grow() or the surrounding logic change in
the future.

Fixes: 1b0f21f0555c "Implementing store support for EVP_SKEY"
CLA: trivial

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
MergeDate: Tue May 26 10:14:50 2026
(Merged from https://github.com/openssl/openssl/pull/31275)