schema BUGFIX reflect the way YIN parser stores extension's arguments

YIN parser is not able to detect what is the argument in extension
without the definition of the extension which is known as late as during the
compilation process. Therefore if the data come from YIN source, it is
necessary to get the extension's argument before compiling the extension
instance.
diff --git a/src/tree_schema_compile.c b/src/tree_schema_compile.c
index 6155ee4..3c6f8c2 100644
--- a/src/tree_schema_compile.c
+++ b/src/tree_schema_compile.c
@@ -439,6 +439,9 @@
     ext->parent = parent;
     ext->parent_type = parent_type;
 
+    lysc_update_path(ctx, ext->parent_type == LYEXT_PAR_NODE ? (struct lysc_node*)ext->parent : NULL, "{extension}");
+    lysc_update_path(ctx, NULL, ext_p->name );
+
     /* get module where the extension definition should be placed */
     for (u = 0; ext_p->name[u] != ':'; ++u);
     mod = lys_module_find_prefix(ctx->mod_def, ext_p->name, u);
@@ -469,19 +472,56 @@
                             "Extension definition of extension instance \"%s\" not found.", ext_p->name),
                      LY_EVALID);
 
+    /* unify the parsed extension from YIN and YANG sources. Without extension definition, it is not possible
+     * to get extension's argument from YIN source, so it is stored as one of the substatements. Here we have
+     * to find it, mark it with LYS_YIN_ARGUMENT and store it in the compiled structure. */
+    if (ext_p->yin && ext->def->argument) {
+        /* Schema was parsed from YIN and an argument is expected, ... */
+        struct lysp_stmt *stmt = NULL;
+
+        if (ext->def->flags & LYS_YINELEM_TRUE) {
+            /* ... argument was the first XML child element */
+            if (ext_p->child && !(ext_p->child->flags & LYS_YIN_ATTR)) {
+                /* TODO check namespace of the statement */
+                if (!strcmp(ext_p->child->stmt, ext->def->argument)) {
+                    stmt = ext_p->child;
+                }
+            }
+        } else {
+            /* ... argument was one of the XML attributes which are represented as child stmt
+             * with LYS_YIN_ATTR flag */
+            for (stmt = ext_p->child; stmt && (stmt->flags & LYS_YIN_ATTR); stmt = stmt->next) {
+                if (!strcmp(stmt->stmt, ext->def->argument)) {
+                    /* this is the extension's argument */
+                    ext->argument = lydict_insert(ctx->ctx, stmt->arg, 0);
+                    break;
+                }
+            }
+        }
+        if (!stmt) {
+            /* missing extension's argument */
+            LOGVAL(ctx->ctx, LY_VLOG_STR, ctx->path, LYVE_REFERENCE,
+                   "Extension instance \"%s\" misses argument \"%s\".", ext_p->name, ext->def->argument);
+            return LY_EVALID;
+
+        }
+        ext->argument = lydict_insert(ctx->ctx, stmt->arg, 0);
+        stmt->flags |= LYS_YIN_ARGUMENT;
+    }
+
     if (ext->def->plugin && ext->def->plugin->compile) {
-        lysc_update_path(ctx, ext->parent_type == LYEXT_PAR_NODE ? (struct lysc_node*)ext->parent : NULL, "{extension}");
-        lysc_update_path(ctx, NULL, ext_p->name );
         if (ext->argument) {
             lysc_update_path(ctx, (struct lysc_node*)ext, ext->argument);
         }
-        LY_CHECK_RET(ext->def->plugin->compile(ctx, ext_p, ext),LY_EVALID);
+        LY_CHECK_RET(ext->def->plugin->compile(ctx, ext_p, ext), LY_EVALID);
         if (ext->argument) {
             lysc_update_path(ctx, NULL, NULL);
         }
-        lysc_update_path(ctx, NULL, NULL);
-        lysc_update_path(ctx, NULL, NULL);
     }
+    ext_p->compiled = ext;
+
+    lysc_update_path(ctx, NULL, NULL);
+    lysc_update_path(ctx, NULL, NULL);
 
     return LY_SUCCESS;
 }
@@ -6669,6 +6709,9 @@
 
     /* check for invalid substatements */
     for (stmt = ext->child; stmt; stmt = stmt->next) {
+        if (stmt->flags & (LYS_YIN_ATTR | LYS_YIN_ARGUMENT)) {
+            continue;
+        }
         for (u = 0; substmts[u].stmt; ++u) {
             if (substmts[u].stmt == stmt->kw) {
                 break;