libyang CHANGE allow ctx to be NULL and read from another parameter

Fixes CESNET/netopeer2#177

Author: michalvasko

src/context.c

@@ -1534,11 +1534,15 @@ ly_ctx_get_node(struct ly_ctx *ctx, const struct lys_node *start, const char *no
 {
     const struct lys_node *node;
 
-    if (!ctx || !nodeid || ((nodeid[0] != '/') && !start)) {
+    if ((!ctx && !start) || !nodeid || ((nodeid[0] != '/') && !start)) {
         ly_errno = LY_EINVAL;
         return NULL;
     }
 
+    if (!ctx) {
+        ctx = start->module->ctx;
+    }
+
     /* sets error and everything */
     node = resolve_json_nodeid(nodeid, ctx, start, output);
 

src/libyang.h.in

@@ -1347,9 +1347,9 @@ const struct lys_submodule *ly_ctx_get_submodule2(const struct lys_module *main_
  * This function is primarily meant as a utility function to ease getting specific schema nodes
  * directly from context having data_path a const immutable string.
  *
- * @param[in] ctx Context to work in.
+ * @param[in] ctx Context to work in. Must be set if \p start is NULL.
  * @param[in] start Starting node for a relative schema node identifier, in which
- * case it is mandatory.
+ * case it is mandatory and \p ctx can be NULL.
  * @param[in] data_path JSON schema node identifier.
  * @param[in] output Search operation output instead input.
  * @return Resolved schema node or NULL.