(note about log: I added an extra line for logging to backend/rdp/output.c, which is why the line number above is off by one. The extra logging doesn't show when running sway because I didn't change the log level)
If the fix is just to return true if there aren't any rects to send (and thus not call rfx_compose_message), then I can file a PR for that. That seems to fix this crash. I don't understand the RDP protocol or freeRDP well enough to know whether this is the correct fix though.