Raise an error if path is compacted during mapping

[radarhere]

Within path_map(), a Python function is called.

Pillow/src/path.c

Lines 393 to 404 in 55a25aa

	xy = self->xy;
	/* apply function to coordinate set */
	for (i = 0; i < self->count; i++) {
	double x = xy[i + i];
	double y = xy[i + i + 1];
	PyObject *item = PyObject_CallFunction(function, "dd", x, y);
	if (!item || !PyArg_ParseTuple(item, "dd", &x, &y)) {
	Py_XDECREF(item);
	return NULL;
	}
	xy[i + i] = x;

That Python function could, theoretically, make a call to ImagePath.Path.compact(), aka path_compact(), reallocating self->xy.

Pillow/src/path.c

Line 300 in 55a25aa

self->xy = realloc(self->xy, 2 * self->count * sizeof(double));

It would then be incorrect for xy to be accessed again by path_map().

I've added a flag to PyPathObject that is set at the start of the mapping loop, and cleared at the end. If the user's custom Python function calls compact() to reallocate the array in the middle, then a ValueError is raised.