Add sanity check to ssl_get_prev_session

Sanity check the |len| parameter to ensure it is positive. Thanks to Kevin
Wojtysiak (Int3 Solutions) and Paramjot Oberoi (Int3 Solutions) for
reporting this issue.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

ssl/ssl_sess.c

@@ -439,7 +439,7 @@ int ssl_get_prev_session(SSL *s, unsigned char *session_id, int len,
     int r;
 #endif
 
-    if (len > SSL_MAX_SSL_SESSION_ID_LENGTH)
+    if (len < 0 || len > SSL_MAX_SSL_SESSION_ID_LENGTH)
         goto err;
 
     if (session_id + len > limit) {