Warn if no <test> nor <edit> elements in <match>

This corrects an error message being reported at
  https://bugs.freedesktop.org/show_bug.cgi?id=71085
  Bug 71085 - "out of memory" errors on empty match element in fonts.conf

and somewhat works as a workaround for
  https://bugs.freedesktop.org/show_bug.cgi?id=59438
  Bug 59438 - Fix <alias> inside <match>