[msitools] wixl: fix crash when Feature.Level isn't specified

[Marc-André Lureau <malureau src gnome org>]

commit 7c9a18465527d889534cc8ca9ed752af39e3a7d7
Author: Marc-André Lureau <marcandre lureau redhat com>
Date:   Fri Jan 11 15:55:54 2019 +0400

    wixl: fix crash when Feature.Level isn't specified
    
    Fixes:
    https://gitlab.gnome.org/GNOME/msitools/issues/7
    
    Signed-off-by: Marc-André Lureau <marcandre lureau redhat com>

 tools/wixl/builder.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/tools/wixl/builder.vala b/tools/wixl/builder.vala
index 695a758..566a73a 100644
--- a/tools/wixl/builder.vala
+++ b/tools/wixl/builder.vala
@@ -471,8 +471,9 @@ namespace Wixl {
             }
 
             string? parent = (feature.parent is WixFeature) ? feature.parent.Id : null;
+            int level = feature.Level != null ? int.parse (feature.Level) : 1;
 
-            db.table_feature.add (feature.Id, display, int.parse (feature.Level), 0, parent, feature.Title, 
feature.Description, feature.ConfigurableDirectory);
+            db.table_feature.add (feature.Id, display, level, 0, parent, feature.Title, feature.Description, 
feature.ConfigurableDirectory);
 
         }