Fix SSL_clear unused return

Fix missing return value check in dtls1_listen when calling SSL_clear().

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/ssl/d1_lib.c b/ssl/d1_lib.c
index 437e89f..ad6561c 100644
--- a/ssl/d1_lib.c
+++ b/ssl/d1_lib.c
@@ -547,7 +547,8 @@
     int ret;
 
     /* Ensure there is no state left over from a previous invocation */
-    SSL_clear(s);
+    if(!SSL_clear(s))
+        return -1;
 
     SSL_set_options(s, SSL_OP_COOKIE_EXCHANGE);
     s->d1->listen = 1;