Fontlist: really ignore invalid fonts

collectFaceInfo() can return an empty table {},
which could cause readerfont.lua's onSetFont()
to crash.

frontend/fontlist.lua

@@ -155,7 +155,7 @@ function FontList:_readList(dir, mark)
             return
         end
         local fi = collectFaceInfo(path)
-        if not fi then return end
+        if not fi or not next(fi) then return end
         fi.change = attr.change
         self.fontinfo[path] = fi
         mark.cache_dirty = true