Fix unchecked return value

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index 8dea1c4..0dd762e 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -2262,8 +2262,8 @@
                              ssl, MBEDTLS_SSL_HS_END_OF_EARLY_DATA,
                              &buf, &buf_len));
 
-    mbedtls_ssl_add_hs_hdr_to_checksum(
-        ssl, MBEDTLS_SSL_HS_END_OF_EARLY_DATA, 0);
+    MBEDTLS_SSL_PROC_CHK(mbedtls_ssl_add_hs_hdr_to_checksum(
+                             ssl, MBEDTLS_SSL_HS_END_OF_EARLY_DATA, 0));
 
     MBEDTLS_SSL_PROC_CHK(
         mbedtls_ssl_finish_handshake_msg(ssl, buf_len, 0));