Fix an @extend edge-case crash.

Closes #1656

doc-src/SASS_CHANGELOG.md

@@ -9,6 +9,8 @@
 
 * `@keyframes` rules are now converted from CSS properly.
 
+* Extending a selector that contains a non-final pseudo-class no longer crashes.
+
 ## 3.4.12 (13 February 2015)
 
 [Tagged on GitHub](https://github.com/sass/sass/releases/tag/3.4.12).

lib/sass/selector/simple.rb

@@ -72,12 +72,9 @@ def eql?(other)
       def unify(sels)
         return sels if sels.any? {|sel2| eql?(sel2)}
         sels_with_ix = Sass::Util.enum_with_index(sels)
-        _, i =
-          if is_a?(Pseudo)
-            sels_with_ix.find {|sel, _| sel.is_a?(Pseudo) && (sels.last.type == :element)}
-          else
-            sels_with_ix.find {|sel, _| sel.is_a?(Pseudo)}
-          end
+        if !is_a?(Pseudo) || (sels.last.is_a?(Pseudo) && sels.last.type == :element)
+          _, i = sels_with_ix.find {|sel, _| sel.is_a?(Pseudo)}
+        end
         return sels + [self] unless i
         sels[0...i] + [self] + sels[i..-1]
       end

test/sass/extend_test.rb

@@ -1338,6 +1338,16 @@ def test_optional_extend_succeeds_when_extension_fails
 
   # Regression Tests
 
+  def test_extend_with_middle_pseudo
+    assert_equal(<<CSS, render(<<SCSS))
+.btn:active.focus, :active.focus:before {
+  a: b; }
+CSS
+.btn:active.focus {a: b}
+:before {@extend .btn}
+SCSS
+  end
+
   def test_extend_parent_selector_suffix
     assert_equal <<CSS, render(<<SCSS)
 .a-b, .c {