Fix memory leak in get_str_from_file()

Fix memory leak in get_str_from_file()

If BIO_gets encounters an empty file or read error, the function
returns NULL without freeing the dynamically allocated heap block (buf).

Safely clear and free the allocated buffer before returning NULL on
the error path. Since get_str_from_file() may handle cryptographic
keys, OPENSSL_clear_free() is used to prevent leaking sensitive data.

Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Norbert Pocs <norbertp@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/30373)

(cherry picked from commit d286240daa3206c0e7d8b49f0dd6b035170fc41e)