Fixing stack smashing bug due to invalid array access

[Zpeugh]

I was getting the following error with an input DEM:

performing terra meshing...
starting greedy insertion with raster width: 1600, height: 1600
*** stack smashing detected ***: <unknown> terminated

Here is the input tif to reproduce.
dem.tif.zip

And you just call it with

tin-terrain dem2tin --input dem.tif --output mesh.obj

This PR should fix this.