[glom] Avoid a null shared_ptr dereference.

[Murray Cumming <murrayc src gnome org>]

commit 61a749a9e12453bb92bb6e9b4dc8fd37effadd71
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Nov 7 13:17:11 2016 +0100

    Avoid a null shared_ptr dereference.
    
    Found by clang-tidy.

 glom/mode_data/db_adddel/db_adddel.cc |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/glom/mode_data/db_adddel/db_adddel.cc b/glom/mode_data/db_adddel/db_adddel.cc
index c558094..a1a5c0b 100644
--- a/glom/mode_data/db_adddel/db_adddel.cc
+++ b/glom/mode_data/db_adddel/db_adddel.cc
@@ -425,6 +425,10 @@ bool DbAddDel::select_item(const Gtk::TreeModel::iterator& iter, bool start_edit
       break;
   }
 
+  if (!layout_item) {
+    return;
+  }
+
   return select_item(iter, *layout_item, start_editing);
 }