tightvnc-filetransfer: fix heap use-after-free

One can only guess what the intended semantics were here, but as every
other rfbCloseClient() call in this file is followed by an immediate
return, let's assume this was forgotton in this case.

Anyway, don't forget to clean up to not leak memory.

Closes #241

libvncserver/tightvnc-filetransfer/handlefiletransferrequest.c

@@ -585,6 +585,8 @@ HandleFileDownloadCancelRequest(rfbClientPtr cl, rfbTightClientPtr rtcp)
 					"FileDownloadCancelMsg\n", __FILE__, __FUNCTION__);
 		
 	    rfbCloseClient(cl);
+	    free(reason);
+	    return;
 	}
 
 	rfbLog("File [%s]: Method [%s]: File Download Cancel Request received:"