jsonrpc: Make sure we handle the case where jcon outlives command

So far only happens during normal shutdown, but it may happen in other
cases as well. We simply define a new destructor that unregisters the
`cmd` from the `jcon`.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/jsonrpc.c

@@ -25,6 +25,7 @@ struct json_output {
 	const char *json;
 };
 
+/* jcon and cmd have separate lifetimes: we detach them on either destruction */
 static void destroy_jcon(struct json_connection *jcon)
 {
 	log_debug(jcon->log, "Closing (%s)", strerror(errno));
@@ -36,6 +37,12 @@ static void destroy_jcon(struct json_connection *jcon)
 	tal_free(jcon->log);
 }
 
+static void destroy_cmd(struct command *cmd)
+{
+	if (cmd->jcon)
+		cmd->jcon->current = NULL;
+}
+
 static void json_help(struct command *cmd,
 		      const char *buffer, const jsmntok_t *params);
 
@@ -460,6 +467,7 @@ static void parse_request(struct json_connection *jcon, const jsmntok_t tok[])
 	jcon->current->id = tal_strndup(jcon->current,
 					json_tok_contents(jcon->buffer, id),
 					json_tok_len(id));
+	tal_add_destructor(jcon->current, destroy_cmd);
 
 	if (!method || !params) {
 		command_fail(jcon->current, method ? "No params" : "No method");