Return zero from ImagePath compact() if there are no points to remove

[radarhere]

Creating an ImagePath with zero points, and then calling compact() should return the number of points that were removed in the compacting process.

from PIL import ImagePath
p = ImagePath.Path(0)
print(p.compact())

Instead this gives -1, as it is presumed that is at least 1 point in the path.

Pillow/src/path.c

Line 336 in e62e2e9

for (i = j = 1; i < self->count; i++) {

Pillow/src/path.c

Line 345 in e62e2e9

i = self->count - j;

Pillow/src/path.c

Line 355 in e62e2e9

return Py_BuildValue("i", i); /* number of removed vertices */

Instead, let's return early, saying that zero points were removed.