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 #25238)

Author: ifranzki

crypto/hmac/hmac_s390x.c

@@ -263,6 +263,7 @@ int s390x_HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx)
     memcpy(&dctx->plat.s390x.param, &sctx->plat.s390x.param,
            sizeof(dctx->plat.s390x.param));
 
+    OPENSSL_clear_free(dctx->plat.s390x.buf, dctx->plat.s390x.size);
     dctx->plat.s390x.buf = NULL;
     if (sctx->plat.s390x.buf != NULL) {
         dctx->plat.s390x.buf = OPENSSL_memdup(sctx->plat.s390x.buf,