Fix for issue 1864 (rebased) (#1866)
Fix for issue 1864 (rebased) (#1866)

* Fix for issue 1864. Segfault reading bad scene file.

The Importer was reading a sceneNode from YAML::Load as a Yaml Map without checking for it being a Map first, causing the error.

* Exit early from scene import if not a valid YAML map

tangrams/tangram-es,tangrams/tangram-es,cleeus/tangram-es,cleeus/tangram-es,tangrams/tangram-es,tangrams/tangram-es,tangrams/tangram-es,cleeus/tangram-es,cleeus/tangram-es,tangrams/tangram-es,cleeus/tangram-es,cleeus/tangram-es,tangrams/tangram-es