s390x: Fix memory leak in s390x_HMAC_CTX_copy()

When s390x_HMAC_CTX_copy() is called, but the destination context already
has a buffer allocated, it is not freed before duplicating the buffer from
the source context.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/25238)