schannel: on connection close there might not be a transfer

Reported-by: Marcel Raad
Fixes #3412
Closes #3483

lib/vtls/schannel.c

@@ -7,7 +7,7 @@
  *
  * Copyright (C) 2012 - 2016, Marc Hoersken, <info@marc-hoersken.de>
  * Copyright (C) 2012, Mark Salisbury, <mark.salisbury@hp.com>
- * Copyright (C) 2012 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 2012 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -2013,9 +2013,16 @@ static int Curl_schannel_shutdown(struct connectdata *conn, int sockindex)
 
   /* free SSPI Schannel API credential handle */
   if(BACKEND->cred) {
-    Curl_ssl_sessionid_lock(conn);
+    /*
+     * When this function is called from Curl_schannel_close() the connection
+     * might not have an associated transfer so the check for conn->data is
+     * necessary.
+     */
+    if(conn->data)
+      Curl_ssl_sessionid_lock(conn);
     Curl_schannel_session_free(BACKEND->cred);
-    Curl_ssl_sessionid_unlock(conn);
+    if(conn->data)
+      Curl_ssl_sessionid_unlock(conn);
     BACKEND->cred = NULL;
   }