[openssl.org #2072] Avoid potential doublefree and reuse of freed handshake_buffer

[Tomas Mraz via RT]

The attached patch against 1.0.0 fixes a potential doublefree and reuse
of freed handshake_buffer when SSL_clear() is called.
-- 
Tomas Mraz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb


--- s3_lib.c.hbuf-clear	2009-05-28 20:10:47.000000000 +0200
+++ s3_lib.c	2009-10-16 09:50:24.000000000 +0200
@@ -2211,6 +2211,7 @@ void ssl3_clear(SSL *s)
  	wlen = s->s3->wbuf.len;
 	if (s->s3->handshake_buffer) {
 		BIO_free(s->s3->handshake_buffer);
+		s->s3->handshake_buffer = NULL;
 	}
 	if (s->s3->handshake_dgst) {
 		ssl3_free_digest_list(s);