[libgda] GdaBrowser: fix segfault set busy parent class

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit ff3eceb75916c1694bdd994c532754e8eedc020f
Author: Daniel Espinosa <esodan gmail com>
Date:   Mon Aug 6 17:20:19 2018 -0500

    GdaBrowser: fix segfault set busy parent class
    
    Trying to update metadata on a PostgreSQL provider, segfaults
    at update metadata

 tools/common/t-virtual-connection.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/tools/common/t-virtual-connection.c b/tools/common/t-virtual-connection.c
index 4b8786c16..3a193f88d 100644
--- a/tools/common/t-virtual-connection.c
+++ b/tools/common/t-virtual-connection.c
@@ -120,8 +120,8 @@ m_busy (TConnection *bcnc, gboolean is_busy, const gchar *reason)
                                                           bcnc);                       
                }
        }
-
-       T_CONNECTION_CLASS (parent_class)->busy (bcnc, is_busy, reason);
+  if (T_CONNECTION_CLASS (parent_class)->busy != NULL)
+    T_CONNECTION_CLASS (parent_class)->busy (bcnc, is_busy, reason);
 }
 
 static void