This fixes a warning with stringop-truncation:
```
error: 'strncpy' output may be truncated copying 127 bytes from a string of length 255 [-Werror=stringop-truncation]
1443 | strncpy(graph->name, s, sizeof(graph->name)-1);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
Signed-off-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18613>