Tiny frontend crashes on SVGs with empty path segments

Example reproducer:
```svg
<svg height="210" width="400" xmlns="http://www.w3.org/2000/svg">
  <path d="M150 5 L75 200 L225 200 Z M100 50"
  style="fill:none;stroke:green;stroke-width:3" />
</svg>
```

Cause: freetype does not handle this edge case well, need some workaround in plot_path.