[gitg/vala] Fix critical

[Ignacio Casal Quinteiro <icq src gnome org>]

commit aa4432a5b205a4db8679ce43ab7d078e45003784
Author: Ignacio Casal Quinteiro <icq gnome org>
Date:   Sat Jul 7 11:58:19 2012 +0200

    Fix critical

 libgitg/gitg-commit-model.vala |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/libgitg/gitg-commit-model.vala b/libgitg/gitg-commit-model.vala
index c5969ec..a0db933 100644
--- a/libgitg/gitg-commit-model.vala
+++ b/libgitg/gitg-commit-model.vala
@@ -126,8 +126,11 @@ public class CommitModel : Object
 			walk.end(res);
 
 			d_cancellable.cancel();
-			d_thread.join();
-			d_thread = null;
+			if (d_thread != null)
+			{
+				d_thread.join();
+				d_thread = null;
+			}
 		});
 	}