Release tracked shared_ptrs outside the lock to avoid a deadlock if
boost::signals2::mutex doesn't lock recursively.
Index: slot_call_iterator.hpp
===================================================================
--- slot_call_iterator.hpp
+++ slot_call_iterator.hpp
@@ -113,8 +113,8 @@
           }
           for(;iter != end; ++iter)
           {
+            cache->tracked_ptrs.clear();
             lock_type lock(**iter);
-            cache->tracked_ptrs.clear();
             (*iter)->nolock_grab_tracked_objects(std::back_inserter(cache->tracked_ptrs));
             if((*iter)->nolock_nograb_connected())
             {