Attempt to fix client curl crash described in #831

* Cherry-picked curl change from curl/curl@927a5bd
- Potentially two more commits that have to be cherry-picked into our build from curl master to completely wipe out this crash, but we will see.
- More information at issue #831

Author: patrikjuvonen

vendor/curl/lib/url.c

@@ -788,19 +788,18 @@ CURLcode Curl_disconnect(struct Curl_easy *data,
     /* This is set if protocol-specific cleanups should be made */
     conn->handler->disconnect(conn, dead_connection);
 
-    /* unlink ourselves! */
   infof(data, "Closing connection %ld\n", conn->connection_id);
+  Curl_ssl_close(conn, FIRSTSOCKET);
+  Curl_ssl_close(conn, SECONDARYSOCKET);
+
+  /* unlink ourselves! */
   Curl_conncache_remove_conn(data, conn, TRUE);
 
   free_idnconverted_hostname(&conn->host);
   free_idnconverted_hostname(&conn->conn_to_host);
   free_idnconverted_hostname(&conn->http_proxy.host);
   free_idnconverted_hostname(&conn->socks_proxy.host);
 
-  /* this assumes that the pointer is still there after the connection was
-     detected from the cache */
-  Curl_ssl_close(conn, FIRSTSOCKET);
-
   conn_free(conn);
   return CURLE_OK;
 }

vendor/curl/lib/vtls/schannel.c

@@ -1960,6 +1960,8 @@ static int Curl_schannel_shutdown(struct connectdata *conn, int sockindex)
   char * const hostname = SSL_IS_PROXY() ? conn->http_proxy.host.name :
     conn->host.name;
 
+  DEBUGASSERT(data);
+  
   infof(data, "schannel: shutting down SSL/TLS connection with %s port %hu\n",
         hostname, conn->remote_port);
 
@@ -2035,11 +2037,9 @@ static int Curl_schannel_shutdown(struct connectdata *conn, int sockindex)
      * might not have an associated transfer so the check for conn->data is
      * necessary.
      */
-    if(conn->data)
-      Curl_ssl_sessionid_lock(conn);
+    Curl_ssl_sessionid_lock(conn);
     Curl_schannel_session_free(BACKEND->cred);
-    if(conn->data)
-      Curl_ssl_sessionid_unlock(conn);
+    Curl_ssl_sessionid_unlock(conn);
     BACKEND->cred = NULL;
   }