[libsigc++2] trackable: move constructor: null the src.

[Murray Cumming <murrayc src gnome org>]

commit 4d7f09efb6ca0bbfe41e99cadaa9ac32e96da380
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Sep 1 16:10:14 2015 +0200

    trackable: move constructor: null the src.

 sigc++/trackable.cc |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/sigc++/trackable.cc b/sigc++/trackable.cc
index e261697..16f68b5 100644
--- a/sigc++/trackable.cc
+++ b/sigc++/trackable.cc
@@ -41,6 +41,7 @@ trackable::trackable(const trackable& /*src*/)
 trackable::trackable(trackable&& src)
 : callback_list_(std::move(src.callback_list_))
 {
+  src.callback_list_ = nullptr;
 }
 
 trackable& trackable::operator=(const trackable& src)