Fix uninitialized access

Found using Codenomicon Defensics.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 09431ce..c7fb0ab 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -410,6 +410,8 @@
     int ret;
     mbedtls_ssl_session session;
 
+    mbedtls_ssl_session_init( &session );
+
     if( ssl->conf->f_ticket_parse == NULL ||
         ssl->conf->f_ticket_write == NULL )
     {