Test w/ NULL pointer to enabled_... object added; bug fixed.

[SVN r16402]

enable_shared_from_this_test.cpp

@@ -66,9 +66,12 @@ void test()
     BOOST_TEST(py.use_count() == 3);
 }
 
+void test2();
+
 int main()
 {
     test();
+    test2();
     return boost::report_errors();
 }
 
@@ -102,3 +105,8 @@ boost::shared_ptr<Y> createY()
     boost::shared_ptr<Y> pi(new impl2);
     return pi;
 }
+
+void test2()
+{
+    boost::shared_ptr<Y> pi(static_cast<impl2*>(0));
+}