[retry] Fix potential use-after-free in timer_expired()

timer->refcnt is allowed to be NULL, in which case the timer's
expired() method may end up freeing the timer object.

Discovered using valgrind.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/retry.c

  * @v timer		Retry timer
  */
 static void timer_expired ( struct retry_timer *timer ) {
+	struct refcnt *refcnt = timer->refcnt;
 	int fail;
 
 	/* Stop timer without performing RTT calculations */
 
 	/* Call expiry callback */
 	timer->expired ( timer, fail );
+	/* If refcnt is NULL, then timer may already have been freed */
 
-	ref_put ( timer->refcnt );
+	ref_put ( refcnt );
 }
 
 /**