lightningd: handle properly if our own request to plugin is freed.

We should really unify the cases of a local request, vs a forwarded
request, but for now, don't steal the request onto the plugin, and
if we return from the plugin and the request is gone, don't get upset.

This uncovered a case where we weren't inside a transaction, in
test_hook_crash, so fix that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/plugin.h

@@ -73,7 +73,7 @@ struct plugin {
 	/* An array of subscribed topics */
 	char **subscriptions;
 
-	/* Currently pending requests by their request ID */
+	/* Our pending requests by their request ID */
 	STRMAP(struct jsonrpc_request *) pending_requests;
 
 	/* If set, the plugin is so important that if it terminates early,
@@ -322,9 +322,13 @@ void plugins_notify(struct plugins *plugins,
 
 /**
  * Send a jsonrpc_request to the specified plugin
+ * @plugin: the plugin to send the request to
+ * @req: the request.
+ *
+ * If @req is freed, any response from the plugin is ignored.
  */
 void plugin_request_send(struct plugin *plugin,
-			 struct jsonrpc_request *req TAKES);
+			 struct jsonrpc_request *req);
 
 /**
  * Callback called when parsing options. It just stores the value in