Don't crash serializing an unknown rule with an unmatched closing brace

[eeshsaxena]

Parsing then serializing certain malformed at-rules raises IndexError from the serializer:

import cssutils
cssutils.parseString('@keyframes k { 0% { opacity: 0; }"}').cssText
# IndexError: pop from empty list   (serialize.py, do_CSSUnknownRule)

do_CSSUnknownRule walks the rule's token sequence keeping a stack of open blocks and pops it on every }. A } with no matching { (which the tokenizer can leave in the sequence for a malformed at-rule) makes it pop from an empty list. I guard the pop with and stacks; a stray } then falls through and is emitted as a plain token. Well-formed unknown rules (including nested blocks) serialize exactly as before.

Added a test to TestCSSUnknownRule; it raises IndexError on main and passes with the change, and the unknown-rule and serialize suites still pass. Found it by fuzzing parseString + cssText.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Don't crash serializing an unknown rule with an unmatched closing brace), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining cssutils!