mprisplugin: Don't skip if size is invalid and take at least one image
The isValid() check was supposed to take the first artwork it finds
but a default-constructed QSize has zero size, which is a valid size.
Moreover, if we failed to parse the size, we won't be bigger than a
valid size it found before, so the continue is redundant and would
cause it not to take the first artwork it found.
Makes artwork show up on Soundcloud which have a bug in their metadata, with a bogus t in the sizes:
