[gnome-mines] Fixed segfault on game close before game start

[Robert Roth <robertroth src gnome org>]

commit ea09b215f4a85bf3d2e6632a8d800be35626f32a
Author: Robert Roth <robert roth off gmail com>
Date:   Wed Jul 2 01:59:46 2014 +0300

    Fixed segfault on game close before game start

 src/minefield-view.vala |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/minefield-view.vala b/src/minefield-view.vala
index e83bb9b..bb21d1e 100644
--- a/src/minefield-view.vala
+++ b/src/minefield-view.vala
@@ -389,12 +389,12 @@ public class MinefieldView : Gtk.Grid
 
     public override void get_preferred_width (out int minimum, out int natural)
     {
-        minimum = natural = (int) (minefield.width * minimum_size);
+        minimum = natural = minefield != null ? (int) (minefield.width * minimum_size) : 0;
     }
 
     public override void get_preferred_height (out int minimum, out int natural)
     {
-        minimum = natural = (int) (minefield.height * minimum_size);
+        minimum = natural = minefield != null ? (int) (minefield.height * minimum_size) : 0;
     }
 
     public new void add (Gtk.Widget child, int i, int j)