Silence uninitialised instance var warning.

Use `haml_buffer` method instead of directly accessing `@haml_buffer`
and check for `@haml_buffer` being defined in method.

Use explicit `haml_buffer` method rather than `attr` as the latter
generates `private attribute?` warning.

lib/haml/compiler.rb

@@ -11,7 +11,7 @@ module Haml
       preamble = <<END.gsub("\n", ";")
 begin
 extend Haml::Helpers
-_hamlout = @haml_buffer = Haml::Buffer.new(@haml_buffer, #{options_for_buffer.inspect})
+_hamlout = @haml_buffer = Haml::Buffer.new(haml_buffer, #{options_for_buffer.inspect})
 _erbout = _hamlout.buffer
 __in_erb_template = true
 END

lib/haml/helpers.rb

@@ -70,7 +70,7 @@ MESSAGE
     #     context.haml_tag :p, "Stuff"
     #
     def init_haml_helpers
-      @haml_buffer = Haml::Buffer.new(@haml_buffer, Haml::Engine.new('').send(:options_for_buffer))
+      @haml_buffer = Haml::Buffer.new(haml_buffer, Haml::Engine.new('').send(:options_for_buffer))
       nil
     end
 
@@ -599,7 +599,7 @@ MESSAGE
     #
     # @return [Haml::Buffer]
     def haml_buffer
-      @haml_buffer
+      @haml_buffer if defined? @haml_buffer
     end
 
     # Gives a proc the same local `_hamlout` and `_erbout` variables