Error 500 on invalid size option

When requesting an avatar with a non-integer size such as the word “mille” (or anything else) ivatar seems to crash and returns a HTTP 500 error code.

Example:

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

Issue found with my tests script ivatar.t.