cssparser: Don't allow commit_token() on block EOF

cssparser: Don't allow commit_token() on block EOF

When we're at the end of a block and gtk_css_parser_get_token() returns
NULL, gtk_css_parser_commit_token() still consumed the next token.

It does not anymore.

This does not affect the CSS parser, but it exposes issues with the
render parser, which previously just consumed too many closing } tokens
in the past.