lib: prevent gRPC assert on missing YANG node

`yang_dnode_get` will `assert` if no YANG node/model exist, so lets test for
its existence first before trying to access it.

This `assert` is only acceptable for internal FRR usage otherwise we
might miss typos or unmatching YANG models nodes/leaves. For gRPC usage
we should let users attempt to use non existing models without
`assert`ing.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>

lib/northbound_grpc.cpp

@@ -344,6 +344,10 @@ static struct lyd_node *get_dnode_config(const std::string &path)
 {
 	struct lyd_node *dnode;
 
+	if (!yang_dnode_exists(running_config->dnode,
+			       path.empty() ? NULL : path.c_str()))
+		return NULL;
+
 	dnode = yang_dnode_get(running_config->dnode,
 			       path.empty() ? NULL : path.c_str());
 	if (dnode)