[smuxi] Engine: fixed NullReferenceException thrown in SqliteMessageBuffer.Count (closes: #1022)

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit b6a0133e0b1a3966c3287d8e4eabd9cff6fa45c5
Author: Mirco Bauer <meebey meebey net>
Date:   Sun May 3 23:21:22 2015 +0200

    Engine: fixed NullReferenceException thrown in SqliteMessageBuffer.Count (closes: #1022)
    
    There can be a race between SqliteMessageBuffer.Dispose() and .get_Count which
    led to this exception:
    
        Server stack trace:
          at Smuxi.Engine.SqliteMessageBuffer.get_Count () [0x00000] in 
/usr/local/src/smuxi/src/Engine/MessageBuffers/SqliteMessageBuffer.cs:42
          at Smuxi.Engine.ChatModel.GetSyncMessages () [0x0000d] in 
/usr/local/src/smuxi/src/Engine/Chats/ChatModel.cs:248
          at Smuxi.Engine.ChatModel.get_Messages () [0x00000] in 
/usr/local/src/smuxi/src/Engine/Chats/ChatModel.cs:82

 src/Engine/MessageBuffers/SqliteMessageBuffer.cs |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/src/Engine/MessageBuffers/SqliteMessageBuffer.cs 
b/src/Engine/MessageBuffers/SqliteMessageBuffer.cs
index f156836..cdca285 100644
--- a/src/Engine/MessageBuffers/SqliteMessageBuffer.cs
+++ b/src/Engine/MessageBuffers/SqliteMessageBuffer.cs
@@ -1,6 +1,6 @@
 // Smuxi - Smart MUltipleXed Irc
 //
-// Copyright (c) 2014 Mirco Bauer <meebey meebey net>
+// Copyright (c) 2014-2015 Mirco Bauer <meebey meebey net>
 //
 // Full GPL License: <http://www.gnu.org/licenses/gpl.txt>
 //
@@ -39,7 +39,11 @@ namespace Smuxi.Engine
 
         public override int Count {
             get {
-                using (var cmd = Connection.CreateCommand()) {
+                var connection = Connection;
+                if (connection == null) {
+                    return 0;
+                }
+                using (var cmd = connection.CreateCommand()) {
                     cmd.CommandText = "SELECT COUNT(*) FROM Messages";
                     return (int) Convert.ChangeType(cmd.ExecuteScalar(), typeof(int));
                 }