Fix a crash that will hit when an image asset is truncated in storage

OpenSim/Region/ClientStack/LindenUDP/J2KImage.cs

@@ -103,8 +103,18 @@ namespace OpenSim.Region.ClientStack.LindenUDP
         {
             if (!m_decoded)
                 return 0;
+            try
+            {
                 return (ushort)(((m_asset.Data.Length - cFirstPacketSize + cImagePacketSize - 1) / cImagePacketSize) + 1);
             }
+            catch(Exception e)
+            {
+                // If the asset is missing/destroyed/truncated, we will land
+                // here
+                //
+                return 0;
+            }
+        }
 
         public void J2KDecodedCallback(UUID AssetId, OpenJPEG.J2KLayerInfo[] layers)
         {