Skip UUIDs (that can't be) read from damaged assets

[appurist]

Reported by Vinhold as a null reference exception when save oar was called with a 1.

[appurist]

2017-03-13 08:19:30,196 [ConsoleThread] ERROR OpenSim.Region.Framework.Scenes.UuidGatherer - [ASSET GATHERER]: Failed to get part - System.NullReferenceException: Object reference not set to an instance of an object.
   at OpenSim.Region.Framework.Scenes.UuidGatherer.GatherAssetUuids(SceneObjectGroup sceneObject, IDictionary`2 assetUuids) in X:\halcyon-0.9.32.6272\OpenSim\Region\Framework\Scenes\UuidGatherer.cs:line 117
   at OpenSim.Region.Framework.Scenes.UuidGatherer.GetSceneObjectAssetUuids(UUID sceneObjectUuid, IDictionary`2 assetUuids) in X:\halcyon-0.9.32.6272\OpenSim\Region\Framework\Scenes\UuidGatherer.cs:line 287
   at OpenSim.Region.Framework.Scenes.UuidGatherer.GatherAssetUuids(UUID assetUuid, AssetType assetType, IDictionary`2 assetUuids) in X:\halcyon-0.9.32.6272\OpenSim\Region\Framework\Scenes\UuidGatherer.cs:line 98
   at OpenSim.Region.Framework.Scenes.UuidGatherer.GatherAssetUuids(SceneObjectGroup sceneObject, IDictionary`2 assetUuids) in X:\halcyon-0.9.32.6272\OpenSim\Region\Framework\Scenes\UuidGatherer.cs:line 170
2017-03-13 08:19:30,212 [ConsoleThread] DEBUG OpenSim.Region.Framework.Scenes.UuidGatherer - [ASSET GATHERER]: Texture entry length for prim was 98 (min is 46)
2017-03-13 08:19:30,227 [ConsoleThread] ERROR OpenSim.Region.Framework.Scenes.Serialization.SceneObjectSerializer - [SERIALIZER]: Deserialization of xml failed with System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
   at System.Xml.XmlTextReaderImpl.Throw(String res, String arg)
   at System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace()
   at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
   at System.Xml.XmlDocument.Load(XmlReader reader)
   at System.Xml.XmlDocument.LoadXml(String xml)
   at OpenSim.Region.Framework.Scenes.Serialization.SceneObjectSerializer.FromOriginalXmlFormat(UUID fromUserInventoryItemID, String xmlData) in X:\halcyon-0.9.32.6272\OpenSim\Region\Framework\Scenes\Serialization\SceneObjectSerializer.cs:line 139.