Recast: fix crash caused by null navmesh

Make sure there is a navmesh before attempting to build static pathing data. The crash gets triggered when loading geometry, and trying to build a tile without generating the navmesh first.

src/naveditor/Editor.cpp

@@ -344,6 +344,8 @@ void Editor::handleUpdate(const float dt)
 
 void Editor::buildStaticPathingData()
 {
+	if (!m_navMesh) return;
+
 	dtDisjointSet data;
 
 	if (!dtCreateDisjointPolyGroups(m_navMesh, data))