Fix crash when using covergrid + default art size

[afontenot]

The default config value for browsers.cover_size is -1. The new covergrid code works fine when the config value is absent, but with this default it will crash after requesting a negative size from GdkPixbuf.Pixbuf.new_from_file_at_size.

[declension]

Is this fixing a documented issue / or have you seen this crash yourself?

[afontenot]

Yes, I hit this myself - immediate crash when switching to covergrid. Probably not many people have hit it because it doesn't affect any released version, it's the new covergrid that's at fault. My config file contains the following:

cover_size = -1

The existing code for this config.getint("browsers", "cover_size", 48) returns -1, not 48, because the value in the config file is an integer. If you look at every other place cover_size is used in the code, it's sanity-checked in exactly the way I'm doing it in this PR.

[declension]

Thanks @afontenot