ext_transform_feedback: prevent possible overflow

ext_transform_feedback: prevent possible overflow

```
/home/okias/projects/collabora/piglit/tests/spec/ext_transform_feedback/max-varyings.c:208:48: error: ‘sprintf’ may write a terminating nul past the end of the destination [-Werror=format-overflow=]
  208 |                         sprintf(buf, "v[%d][%d]", i, j);
      |                                                ^
/home/okias/projects/collabora/piglit/tests/spec/ext_transform_feedback/max-varyings.c:208:25: note: ‘sprintf’ output between 8 and 17 bytes into a destination of size 16
  208 |                         sprintf(buf, "v[%d][%d]", i, j);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/726>