Raise ValueError for BoxBlur filter with negative radius

[hugovk]

Fixes #6873.

Changes proposed in this pull request:

• There's a segmentation fault when using a filter with a negative radius
• A radius of zero has a special meaning (do not blur, returns identical image)
• https://pillow.readthedocs.io/en/stable/reference/ImageFilter.html#PIL.ImageFilter.BoxBlur
• So let's raise a ValueError when creating a filter with a negative radius
• We could raise it when using the filter, but why not already do so when creating?