Assertion error on jcv_diagram_generate

[AtsushiSakai]

Hi. first of all, thank you so much for your great OSS. This library is very useful and easy to use. Thanks!!

I might find an issue of this OSS. When I input a certain data into jcv_diagram_generate function, an assertion was reported:

Assertion failed: (internal->numsites == 1), function jcv_fillgaps, file src/jc_voronoi.h, line 1143.

This repo is a fork of your voronoi repo and I added a test code for reproducing the issue. https://github.com/AtsushiSakai/voronoi If you have time, please take a look the file test/assert_text.c file. https://github.com/AtsushiSakai/voronoi/blob/master/test/assert_test.c test/invalid_data.h includes input x-y point data for the issue.

#38 might be a same issue.