If the uuid gatherer fails to find the asset containing gesture metadata, then simply return rather than generating an exception

This matches existing behaviour, though better diagnostics for missing assets may be good later on.
This addresses http://opensimulator.org/mantis/view.php?id=4977

OpenSim/Region/Framework/Scenes/UuidGatherer.cs

@@ -291,9 +291,16 @@ namespace OpenSim.Region.Framework.Scenes
             }
         }
 
+        /// <summary>
+        /// Get the asset uuid associated with a gesture
+        /// </summary>
+        /// <param name="gestureUuid"></param>
+        /// <param name="assetUuids"></param>
         protected void GetGestureAssetUuids(UUID gestureUuid, IDictionary<UUID, AssetType> assetUuids)
         {
             AssetBase assetBase = GetAsset(gestureUuid);
+            if (null == assetBase)
+                return;
 
             MemoryStream ms = new MemoryStream(assetBase.Data);
             StreamReader sr = new StreamReader(ms);