libevent-client: fix Incorrect format found by PVS Studio (V576)

Consider checking the fourth actual argument of the 'fprintf' function. The SIGNED integer type argument is expected.

examples/libevent-client.c

@@ -287,7 +287,7 @@ static int on_stream_close_callback(nghttp2_session *session, int32_t stream_id,
   int rv;
 
   if (session_data->stream_data->stream_id == stream_id) {
-    fprintf(stderr, "Stream %d closed with error_code=%d\n", stream_id,
+    fprintf(stderr, "Stream %d closed with error_code=%u\n", stream_id,
             error_code);
     rv = nghttp2_session_terminate_session(session, NGHTTP2_NO_ERROR);
     if (rv != 0) {