Fix crash when using covergrid + default art size (#4151)

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

quodlibet/browsers/covergrid/main.py

@@ -157,7 +157,10 @@ def do_focus(self, direction):
 
 def _get_cover_size():
     mag = config.getfloat("browsers", "covergrid_magnification", 3.)
-    return mag * config.getint("browsers", "cover_size", 48)
+    size = config.getint("browsers", "cover_size")
+    if size <= 0:
+        size = 48
+    return mag * size
 
 
 class CoverGrid(Browser, util.InstanceTracker, DisplayPatternMixin):