Requests with a negative size option crash ivatar

Example:

curl -i "https://avatars.linux-kernel.at/avatar/4751ed9aae86881d2b45dd0512c3e14a?s=-19"

This produce an uncatched ValueError exception with the following message: “height and width must be > 0”.

My implementation returns an HTTP status code 400 while libravatar returns an image with the default size.