Bug 798133 - Gnucash crash when any custom action is entered

1. Protect strlen call in gnucash_sheet_modify_current_cell to
prevent it from crashing.

2. Handle NULL selection in gnc_item_list_key_event when key is
GDK_KEY_Return so that new string in combo control is handled
correctly.

gnucash/register/register-gnome/gnucash-item-list.c

@@ -352,6 +352,8 @@ gnc_item_list_key_event (GtkWidget* widget, GdkEventKey* event, gpointer data)
     {
     case GDK_KEY_Return:
         string = gnc_item_list_get_selection (item_list);
+        if (!string) // Nothing selected, might be new value
+             break;  // Let the sheet deal with it.
         g_signal_emit (G_OBJECT (item_list),
                        gnc_item_list_signals[ACTIVATE_ITEM],
                        0,

gnucash/register/register-gnome/gnucash-sheet.c

@@ -888,7 +888,7 @@ gnucash_sheet_modify_current_cell (GnucashSheet *sheet, const gchar *new_text)
     GtkEditable *editable;
     Table *table = sheet->table;
     VirtualLocation virt_loc;
-    int new_text_len;
+    int new_text_len = 0;
     const char *retval;
     int cursor_position, start_sel, end_sel;
 
@@ -905,7 +905,8 @@ gnucash_sheet_modify_current_cell (GnucashSheet *sheet, const gchar *new_text)
     cursor_position = gtk_editable_get_position (editable);
     gtk_editable_get_selection_bounds (editable, &start_sel, &end_sel);
 
-    new_text_len = strlen (new_text);
+    if (new_text)
+         new_text_len = strlen (new_text);
 
     retval = gnc_table_modify_update (table, virt_loc,
                                       new_text, new_text_len,