(isc_destroy): Don't chanse a NULL pointer. This could be triggered

	if the port was already bound.

src/libraries/libisc-new/src/isc_session.c

@@ -435,7 +435,7 @@ isc_destroy(struct isc_mcb *mcb,
   if (sci->wr_msg_q)
   {
     size = isc_killqueue(sci->wr_msg_q);
-    if (mcb->write_change_cb)
+    if (mcb != NULL && mcb->write_change_cb)
   	  mcb->write_change_cb(-size);
   }