Multiple calls to Sass::Engine#render shouldn't accumulate results

Additionally: removed the "@line = nil" from #render_to_tree since it seems better suited inside #split_lines
(where it's already present).

Signed-off-by: Nathan Weizenbaum <nex342@gmail.com>

lib/sass/engine.rb

@@ -113,7 +113,7 @@ module Sass
           end
         end
       end
-      @line = nil
+      @lines.clear
 
       root
     end

test/sass/engine_test.rb

@@ -52,6 +52,11 @@ class SassEngineTest < Test::Unit::TestCase
     renders_correctly "basic", { :style => :compact }
   end
 
+  def test_multiple_calls_to_render
+    sass = Sass::Engine.new("a\n  b: c")
+    assert_equal sass.render, sass.render
+  end
+
   def test_alternate_styles
     renders_correctly "expanded", { :style => :expanded }
     renders_correctly "compact", { :style => :compact }