[tracker] tracker-store: Fix crash on client exit and task without destroy notify

[Jürg Billeter <juergbi src gnome org>]

commit 9aa346d125d3ea6bc4d5d6473b2df423a562dd09
Author: Jürg Billeter <j bitron ch>
Date:   Wed Nov 24 17:38:45 2010 +0100

    tracker-store: Fix crash on client exit and task without destroy notify

 src/tracker-store/tracker-store.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/src/tracker-store/tracker-store.c b/src/tracker-store/tracker-store.c
index f68e16a..7e8228b 100644
--- a/src/tracker-store/tracker-store.c
+++ b/src/tracker-store/tracker-store.c
@@ -579,7 +579,10 @@ unreg_task (TrackerStoreTask *task,
 	} else if (task->type == TRACKER_STORE_TASK_TYPE_TURTLE) {
 		task->callback.turtle_callback (error, task->user_data);
 	}
-	task->destroy (task->user_data);
+
+	if (task->destroy) {
+		task->destroy (task->user_data);
+	}
 
 	store_task_free (task);
 }