Disallow zero-length magnet URIs
Disallow zero-length magnet URIs

magnet: would match, but now we have a zero-length lookahead which
requires the following character to be a question mark: magnet:?

chimo/social,foocorp/gnu-social,foocorp/gnu-social,charnimda/lgbtpzn-social,charnimda/lgbtpzn-social,chimo/social,chimo/social,charnimda/lgbtpzn-social,foocorp/gnu-social