Fix messages to `$` prefixed topics being rejected.

Closes #2111. Thanks to yellowgg2.

ChangeLog.txt

@@ -4,6 +4,7 @@
 Broker:
 - Fix `tls_version` behaviour not matching documentation. It was setting the
   exact TLS version to use, not the minimium TLS version to use. Closes #2110.
+- Fix messages to `$` prefixed topics being rejected. Closes #2111.
 
 
 2.0.8 - 2021-02-25

src/retain.c

@@ -83,7 +83,10 @@ int retain__store(const char *topic, struct mosquitto_msg_store *stored, char **
 	assert(split_topics);
 
 	HASH_FIND(hh, db.retains, split_topics[0], strlen(split_topics[0]), retainhier);
-	if(retainhier == NULL) return MOSQ_ERR_NOT_FOUND;
+	if(retainhier == NULL){
+		retainhier = retain__add_hier_entry(NULL, &db.retains, split_topics[0], (uint16_t)strlen(split_topics[0]));
+		if(!retainhier) return MOSQ_ERR_NOMEM;
+	}
 
 	for(i=0; split_topics[i] != NULL; i++){
 		slen = strlen(split_topics[i]);