fix handling of some malformed css

[flinst]

This fixes two incompatibilities between shady-css-parser
and browsers.

1. If the css is missing a closing brace like
   div { color:red
   then Shady drops the last rule, but browsers keep it.

2. If a css ruleset has an empty selector like
   /*empty*/ { color: red }
   div { color: blue }
   then Shady discards the { } and produces this:
   color:red;div { color: blue; }
   which has an entirely different meaning.

   Browsers parse the empty-selector block, but then ignore it
   as invalid.

   For this case, we could either make Shady keep the block
   or drop the block. I decided to keep the block, because
   Shady generally passes through many other not-quite-valid
   css text that browsers would ignore.