[libsigcplusplus] Rearrange another comment.

[Murray Cumming <murrayc src gnome org>]

commit 01f06becf3ca2fdab9e684c73a54d3628d009874
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Mar 31 15:20:08 2016 +0200

    Rearrange another comment.

 sigc++/trackable.cc |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)
---
diff --git a/sigc++/trackable.cc b/sigc++/trackable.cc
index 510ac18..e74133a 100644
--- a/sigc++/trackable.cc
+++ b/sigc++/trackable.cc
@@ -113,10 +113,11 @@ trackable_callback_list::~trackable_callback_list()
 void
 trackable_callback_list::add_callback(notifiable* data, func_destroy_notify func)
 {
-  if (!clearing_) // TODO: Is it okay to silently ignore attempts to add dependencies when the list
-                  // is being cleared?
-    //       I'd consider this a serious application bug, since the app is likely to segfault.
-    //       But then, how should we handle it? Throw an exception? Martin.
+  // TODO: Is it okay to silently ignore attempts to add dependencies when the list
+  // is being cleared?
+  // I'd consider this a serious application bug, since the app is likely to segfault.
+  // But then, how should we handle it? Throw an exception? Martin.
+  if (!clearing_)
     callbacks_.push_back(trackable_callback(data, func));
 }