schema tree REFACTOR compile case like a general node

Change included - all choice data children are not linked
siblings anymore but standard children of their case.
Fixes #1157
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index 0623dbd..f239fe1 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -1358,11 +1358,7 @@
     case LYS_CONTAINER:
         return &((struct lysc_node_container *)node)->child;
     case LYS_CHOICE:
-        if (((struct lysc_node_choice *)node)->cases) {
-            return &((struct lysc_node_choice *)node)->cases->child;
-        } else {
-            return NULL;
-        }
+        return (struct lysc_node **)&((struct lysc_node_choice *)node)->cases;
     case LYS_CASE:
         return &((struct lysc_node_case *)node)->child;
     case LYS_LIST: