Clear state in DTLSv1_listen

This is a backport of commit e83ee04bb7de800cdb71d522fa562e99328003a3 from
the master branch (and this has also been applied to 1.0.2). In 1.0.2 this
was CVE-2015-0207. For other branches there is no known security issue, but
this is being backported as a precautionary measure.

The DTLSv1_listen function is intended to be stateless and processes
the initial ClientHello from many peers. It is common for user code to
loop over the call to DTLSv1_listen until a valid ClientHello is received
with an associated cookie. A defect in the implementation of DTLSv1_listen
means that state is preserved in the SSL object from one invokation to the
next.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/d1_lib.c

@@ -496,6 +496,9 @@ int dtls1_listen(SSL *s, struct sockaddr *client)
 {
     int ret;
 
+    /* Ensure there is no state left over from a previous invocation */
+    SSL_clear(s);
+
     SSL_set_options(s, SSL_OP_COOKIE_EXCHANGE);
     s->d1->listen = 1;