Don't segfault on & {}

[saper]

Eases #1519

Now "& {}" just returns an empty CSS - whereas Ruby
reports:

Error: Base-level rules cannot contain the parent-selector-referencing character '&'.
        on line 1 of /home/saper/sw/nodes-sass-problems/1120/test.scss
  Use --trace for backtrace.

Another example:

foo {
  @extend &;
}

generates this error message:

Error: nested selectors may not be extended

whereas Ruby Sass gives:

Error: Can't extend &: can't extend parent selectors
        on line 2 of /home/saper/sw/nodes-sass-problems/1120/test1.scss
  Use --trace for backtrace.

[saper]

I can't say I fully understand what shall Complex_Selector::parentize() return - I sense this method should fail in some cases right away.

This change just covers up the problem, probably.

[mgreter]

This should be covered by #1527

[saper]

Superseded by #1527