rasterizer: fix pointer arithmetic in assert
rasterizer: fix pointer arithmetic in assert

path->points can be NULL and any pointer arithmetic
on NULL, even NULL + 0, is undefined behaviour.
The rest of the function should be safe with NULL.

Found by AFL++ and UBSAN.

astiob/libass,ShiftMediaProject/libass,ShiftMediaProject/libass,libass/libass,libass/libass,astiob/libass