When a lot of lines are being drawn, this can overflow the vertex buffer, causing a crash in `nvgRenderFlush`. While the underlying code handles this case in an okay manner (truncating the vertex buffer instead of crashing), the `nvgRenderFlush` code is missing a check for this case, causing it to think it has more space than was allocated.
I tried to fix it with multiple buffers as mentioned in #160, however it seems more complex than it appears. This fix just makes it truncate the output according to the actual allocated amount of bytes in the vertex buffer, so it doesn't crash at least.
In our situation, the crash happened due to the view dimensions being reset to 0x0 after minimizing the window. This translated to fillCount of a path ending up at 0, which makes `fan` crash as it computes number of tris to -2 of fillCount. Since `numTris` is unsigned, this could lead to terrible things, but luckily it crashes soon.
There might be a cleaner fix for this further up the call stack, but this is a good solution in the meanwhile.
