[glom] FlowTableWithFields::add_field(): Avoid null pointer dereference.

[Murray Cumming <murrayc src gnome org>]

commit d91678914ba823034c5637813ae3ac53b7bc87d0
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Dec 4 21:00:23 2014 +0100

    FlowTableWithFields::add_field(): Avoid null pointer dereference.
    
    Found by scan-build.

 glom/mode_data/flowtablewithfields.cc |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/glom/mode_data/flowtablewithfields.cc b/glom/mode_data/flowtablewithfields.cc
index bf65381..0406a6e 100644
--- a/glom/mode_data/flowtablewithfields.cc
+++ b/glom/mode_data/flowtablewithfields.cc
@@ -496,7 +496,9 @@ void FlowTableWithFields::add_field(const sharedptr<LayoutItem_Field>& layoutite
   }
 
   Gtk::EventBox* eventbox = Gtk::manage(new Gtk::EventBox());
-  eventbox->add(*info.m_first);
+  if(info.m_first)
+      eventbox->add(*info.m_first);
+
   eventbox->set_halign(Gtk::ALIGN_START);
   info.m_first_eventbox = eventbox; //Remember it so we can retrieve the column number later from FlowTable.
   eventbox->set_visible_window(false);