Set s->d1 to NULL after freeing it.

ssl/d1_lib.c

diff --git a/ssl/d1_lib.c b/ssl/d1_lib.c
index ddbb78a..b70bce6 100644
--- a/ssl/d1_lib.c
+++ b/ssl/d1_lib.c
@@ -222,6 +222,7 @@
 	pqueue_free(s->d1->buffered_app_data.q);
 
 	OPENSSL_free(s->d1);
+	s->d1 = NULL;
 	}
 
 void dtls1_clear(SSL *s)