SSL: fixed $ssl_session_id possible segfault after 97e3769637a7.

Even during execution of a request it is possible that there will be
no session available, notably in case of renegotiation.  As a result
logging of $ssl_session_id in some cases caused NULL pointer dereference
after revision 97e3769637a7 (1.5.9).  The check added returns an empty
string if there is no session available.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 9935fb0..cbe4136 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -2508,6 +2508,10 @@
     SSL_SESSION  *sess;
 
     sess = SSL_get0_session(c->ssl->connection);
+    if (sess == NULL) {
+        s->len = 0;
+        return NGX_OK;
+    }
 
     buf = sess->session_id;
     len = sess->session_id_length;