Throw an error on cross-media @extends

[xzyfer]

Originally reported in #712 and fixed in 3.2.0. This has since regressed.

.foo {
  content: 'foo'; }

@media print {
  .bar {
    @extend .foo; } }

Expected

You may not @extend an outer selector from within @media.
You may only @extend selectors within the same directive.
From "@extend .foo" on line 7.

Result

.foo, .bar {
  content: 'foo'; }

Spec added sass/sass-spec#538