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
```

0 files changed, 0 insertions, 0 deletions