Fix YangSchema::childNodes with paths with choice

YangSchema::childNodes, if called with a path with a choice segfaulted,
because it was using libyang in a different way, than the rest of the
class. childNodes now uses libyang consistenly with the rest of the
class.

Change-Id: I81f19114a3088994c552c1bb2fd9b4e3bc326196
diff --git a/src/yang_schema.hpp b/src/yang_schema.hpp
index 12a250c..aa5faa4 100644
--- a/src/yang_schema.hpp
+++ b/src/yang_schema.hpp
@@ -69,6 +69,11 @@
     /** @short Returns a set of nodes, that match the location and name criteria. */
 
     /** @short Returns a single Schema_Node if the criteria matches only one, otherwise nullptr. */
+    std::shared_ptr<libyang::Schema_Node> getSchemaNode(const std::string& node) const;
+
+    /** @short Returns a single Schema_Node if the criteria matches only one, otherwise nullptr. */
     std::shared_ptr<libyang::Schema_Node> getSchemaNode(const schemaPath_& location, const ModuleNodePair& node) const;
     std::shared_ptr<libyang::Context> m_context;
+
+    std::shared_ptr<libyang::Schema_Node> impl_getSchemaNode(const std::string& node) const;
 };