messages/outbox: Fix undefined var warnings

This code needs urgent refactoring. I just applied something similar to
what other files in this module are doing.

main/messages/outbox.php

@@ -112,6 +112,8 @@ if (isset($_REQUEST['action'])) {
     $action = $_REQUEST['action'];
 }
 
+$social_right_content = '';
+
 if (api_get_setting('allow_social_tool') == 'true') {
     $social_left_content = SocialManager::show_social_menu('messages');
     $social_right_content .= '<div class="span9">';
@@ -153,7 +155,6 @@ if (api_get_setting('allow_social_tool') == 'true') {
 $tpl = new Template(get_lang('ComposeMessage'));
 if (api_get_setting('allow_social_tool') == 'true') {
     $tpl->assign('social_left_content', $social_left_content);
-    $tpl->assign('social_left_menu', $social_left_menu);
     $tpl->assign('social_right_content', $social_right_content);
     $social_layout = $tpl->get_template('layout/social_layout.tpl');
     $tpl->display($social_layout);
@@ -163,4 +164,4 @@ if (api_get_setting('allow_social_tool') == 'true') {
     $tpl->assign('message', $show_message);
     $tpl->assign('content', $content);
     $tpl->display_one_col_template();
-}
+}