Raise ValueError during filter operation as well

[radarhere]

Suggestion for python-pillow#6874

Other ways to trigger the problem are

import numpy as np
from PIL import Image, ImageFilter

img = Image.fromarray(np.ones([100, 100, 3], dtype='uint8'))
blur = ImageFilter.BoxBlur(2)
blur.radius = -2
img = img.filter(blur)

You could also abuse getdata() to access the C api directly, but that's less legitimate.

import numpy as np
from PIL import Image, ImageFilter

img = Image.fromarray(np.ones([100, 100, 3], dtype='uint8'))
img = img.getdata().box_blur(-2)

This PR raises an error from C if a negative radius is used after initialisation of the filter to prevent those scenarios.

[hugovk]

Thanks!

Funny thing is I also tried out this very same C change and with that first test case, but mistakenly thought it was being caught by just the Python change 🙃