Fix for convexFill crash when path fillCount is 0

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.