Fix strange "error":null JSON node

An `error` node with NULL value does not make much of a sense, but let's try to handle it.
Closes #3047

(cherry picked from commit 1a8f8e4)

modules/jsonrpc/jsonrpc.c

@@ -339,7 +339,7 @@ static int jsonrpc_handle_cmd(union sockaddr_union *dst, char *cmd, int *id,
 	}
 
 	aux = cJSON_GetObjectItem(obj, "error");
-	if (aux) {
+	if (aux && aux->type!=cJSON_NULL) {
 		/* return the entire error */
 		vret->rs.s = cJSON_Print(aux);
 		vret->rs.len = strlen(vret->rs.s);