Segfault when generating some indexed paletted PNGs

I'm trying to use canvas to draw some polygons and output an indexed, paletted png. This code works for some sets of polygons but for others I get the error:
```
libpng error: Wrote palette index exceeding num_palette
Segmentation fault (core dumped)
```

I've attached a script that generates this segfault every time on my machine, though on some other machines it just spits out the error and doesn't segfault.

[canvas-error.zip](https://github.com/Automattic/node-canvas/files/2699761/canvas-error.zip)

## Your Environment
* canvas@2.2.0
* libpng 1.6.x
* Environment is Fedora Linux 29