TLS1.2: Check for failures in Finished calculation

If the calc_finished function returns an error code, don't ignore it but
instead return the error code to stop the handshake as the Finished
message may be incorrect.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index a5e5b28..71c834b 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -8357,6 +8357,7 @@
     ret = ssl->handshake->calc_finished(ssl, ssl->out_msg + 4, ssl->conf->endpoint);
     if (ret != 0) {
         MBEDTLS_SSL_DEBUG_RET(1, "calc_finished", ret);
+        return ret;
     }
 
     /*
@@ -8470,6 +8471,7 @@
     ret = ssl->handshake->calc_finished(ssl, buf, ssl->conf->endpoint ^ 1);
     if (ret != 0) {
         MBEDTLS_SSL_DEBUG_RET(1, "calc_finished", ret);
+        return ret;
     }
 
     if ((ret = mbedtls_ssl_read_record(ssl, 1)) != 0) {