Improve interop by not writing ext_len in ClientHello 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 e24494a..c4ddcbd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -49,6 +49,8 @@
    * Some example server programs were not sending the close_notify alert.
    * Potential memory leak in mpi_exp_mod() when error occurs during
      calculation of RR.
+   * Improve interoperability by not writing extension length in ClientHello
+     when no extensions are present (found by Matthew Page)
 
 = Version 1.2.10 released 2013-10-07
 Changes