Fix CSS declaration skip after extra semicolon

[Slayer95]

Fixes #1993

This issue happens because declaration() is called with a token list such as

[";", "margin", ":", "auto"]

The token ; is recognized as an invalid identifier, and the parser advances the token stream until the next semicolon (or closing brace).

With this patch, the parser will advance a single token when meeting either whitespace or a semicolon, thus fixing the issue.

Alternative patches that may be considered:

• Slayer95@37193df
• Slayer95@62a8055
• Checking the previous token in skipDeclaration() ?

[kpreid]

Could you add some test cases to cssparser_test.js?