Improve interop by not writing ext_len in ClientHello / ServerHello when 0

The RFC also indicates that without any extensions, we should write a
struct {} (empty) not an array of length zero.
diff --git a/ChangeLog b/ChangeLog
index 7b14ce8..f3b9de1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,8 @@
    * cert_write app should use subject of issuer certificate as issuer of cert
    * Fix false reject in padding check in ssl_decrypt_buf() for CBC
      ciphersuites, for full SSL frames of data.
+   * Improve interoperability by not writing extension length in ClientHello /
+     ServerHello when no extensions are present (found by Matthew Page)
 
 = PolarSSL 1.3.6 released on 2014-04-11