Disallow parent selectors in selector-append

[glebm]

Now correctly errors on inputs such as this one instead of crashing (see #2663):

div {
  a: selector-append('.menu', 'li', '&');
}

Error: Parent selectors aren't allowed here.
        on line 3:37 of ../../../tmp/test.scss, in function `selector-append`
        from line 3:6 of ../../../tmp/test.scss
>>   e: selector-append('.menu', 'li', '&');

   ------------------------------------^

[nschonni]

Might need a PR to sass-spec for this one. I'm guessing there are already tests that are just disabled if this error wasn't supported before that need to be enabled

[glebm]

No disabled tests get fixed by this so I guess a libsass-closed-issues test should be added to sass-spec.

[xzyfer]

There isn't a sass-spec because there wasn't a reported issue. We were producing an error in this case, but not the right error

Error: $selectors: "&" is not a valid selector: Invalid CSS after "": expected selector, was "&" for `selector-append'
        on line 2 of test.scss
  Use --trace for backtrace

The SEGV from #2663 is a known issue with reparsing selector during eval. I've previously created the selector-validation tag for related issues. The fix for that is a fairly significant refactor.

[glebm]

We were producing an error in this case, but not the right error

We weren't producing an error, we were segfaulting instead:

sassc/bin/sassc ~/tmp/test.scss 
[1]    6924 segmentation fault (core dumped)  sassc/bin/sassc ~/tmp/test.scss