Empty connections can be safely copied now

AIGRIND-LLC · murraycu · commit 138510d4002a · 2019-09-26T11:43:43.000+02:00
Copy constructor and `operator=` could fail if `src` was empty.
```
sigc::connection con1;
sigc::connection con2(con1); // failed
```
```
sigc::connection con3;
sigc::connection con4;
con3 = con4; // failed
```
diff --git a/sigc++/weak_raw_ptr.h b/sigc++/weak_raw_ptr.h
@@ -50,7 +50,8 @@ struct weak_raw_ptr : public sigc::notifiable
   inline weak_raw_ptr(const weak_raw_ptr& src) noexcept
   : p_(src.p_)
   {
-    p_->add_destroy_notify_callback(this, &notify_object_invalidated);
+    if (p_)
+      p_->add_destroy_notify_callback(this, &notify_object_invalidated);
   }
 
   inline weak_raw_ptr& operator=(const weak_raw_ptr& src) noexcept
@@ -60,7 +61,9 @@ struct weak_raw_ptr : public sigc::notifiable
     }
 
     p_ = src.p_;
-    p_->add_destroy_notify_callback(this, &notify_object_invalidated);
+    
+    if (p_)
+      p_->add_destroy_notify_callback(this, &notify_object_invalidated);
 
     return *this;
   }

Original file line number	Diff line number	Diff line change
`@@ -50,7 +50,8 @@ struct weak_raw_ptr : public sigc::notifiable`
`50`	`50`	`inline weak_raw_ptr(const weak_raw_ptr& src) noexcept`
`51`	`51`	`: p_(src.p_)`
`52`	`52`	`{`
`53`		`- p_->add_destroy_notify_callback(this, &notify_object_invalidated);`
	`53`	`+ if (p_)`
	`54`	`+ p_->add_destroy_notify_callback(this, &notify_object_invalidated);`
`54`	`55`	`}`
`55`	`56`
`56`	`57`	`inline weak_raw_ptr& operator=(const weak_raw_ptr& src) noexcept`
`@@ -60,7 +61,9 @@ struct weak_raw_ptr : public sigc::notifiable`
`60`	`61`	`}`
`61`	`62`
`62`	`63`	`p_ = src.p_;`
`63`		`- p_->add_destroy_notify_callback(this, &notify_object_invalidated);`
	`64`	`+`
	`65`	`+ if (p_)`
	`66`	`+ p_->add_destroy_notify_callback(this, &notify_object_invalidated);`
`64`	`67`
`65`	`68`	`return *this;`
`66`	`69`	`}`