Handle errors from functions that now return int

A few functions were changed from returning void to returning int three
commits ago. Make sure their callers check the return values.

This commits was basically a matter of declaring newly-int-returning
functions MBEDTLS_CHECK_RETURN_CRITICAL and then fixing the resulting
warnings. A few functions had to be made int in the process; they were
applied the same process as well.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ssl_tls12_server.c b/library/ssl_tls12_server.c
index 9298292..d5c8b7c 100644
--- a/library/ssl_tls12_server.c
+++ b/library/ssl_tls12_server.c
@@ -1020,7 +1020,11 @@
 
     MBEDTLS_SSL_DEBUG_BUF(4, "record contents", buf, msg_len);
 
-    ssl->handshake->update_checksum(ssl, buf, msg_len);
+    ret = ssl->handshake->update_checksum(ssl, buf, msg_len);
+    if (0 != ret) {
+        MBEDTLS_SSL_DEBUG_RET(1, ("update_checksum"), ret);
+        return ret;
+    }
 
     /*
      * Handshake layer:
@@ -4129,7 +4133,11 @@
     /* Calculate hash and verify signature */
     {
         size_t dummy_hlen;
-        ssl->handshake->calc_verify(ssl, hash, &dummy_hlen);
+        ret = ssl->handshake->calc_verify(ssl, hash, &dummy_hlen);
+        if (0 != ret) {
+            MBEDTLS_SSL_DEBUG_RET(1, ("calc_verify"), ret);
+            return ret;
+        }
     }
 
     if ((ret = mbedtls_pk_verify(peer_pk,
@@ -4139,7 +4147,11 @@
         return ret;
     }
 
-    mbedtls_ssl_update_handshake_status(ssl);
+    ret = mbedtls_ssl_update_handshake_status(ssl);
+    if (0 != ret) {
+        MBEDTLS_SSL_DEBUG_RET(1, ("mbedtls_ssl_update_handshake_status"), ret);
+        return ret;
+    }
 
     MBEDTLS_SSL_DEBUG_MSG(2, ("<= parse certificate verify"));