Fix sometimes uninitialized variable warning raised by Xcode 9.4.1

Fixes this warning raised by Travis CI on macOS:
```
editor/plugins/polygon_2d_editor_plugin.cpp:95:6: warning: variable 'skeleton' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (!node->has_node(node->get_skeleton())) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
editor/plugins/polygon_2d_editor_plugin.cpp:106:7: note: uninitialized use occurs here
        if (!skeleton) {
             ^~~~~~~~
editor/plugins/polygon_2d_editor_plugin.cpp:95:2: note: remove the 'if' if its condition is always false
        if (!node->has_node(node->get_skeleton())) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
editor/plugins/polygon_2d_editor_plugin.cpp:94:22: note: initialize the variable 'skeleton' to silence this warning
        Skeleton2D *skeleton;
                            ^
                             = NULL
```

editor/plugins/polygon_2d_editor_plugin.cpp

@@ -91,7 +91,7 @@ void Polygon2DEditor::_notification(int p_what) {
 
 void Polygon2DEditor::_sync_bones() {
 
-	Skeleton2D *skeleton;
+	Skeleton2D *skeleton = NULL;
 	if (!node->has_node(node->get_skeleton())) {
 		error->set_text(TTR("The skeleton property of the Polygon2D does not point to a Skeleton2D node"));
 		error->popup_centered_minsize();