Fix two segfaults with parent selectors

[mgreter]

Slightly tweaks the way we parse selectors in regard to
the parent selector. We only add one if we are not parsing
inside some root context (passed by boolean argument).

Fixes #1519

test-1

foo {
  @extend &;
}

test-2

& {}

test-3

& foo {
  bar: baz;
}

test-4

foo & {
  bar: baz;
}

[saper]

Yes, thank you!

[xzyfer]

Specs added and activated in sass/sass-spec#689