avoid potencial null ref

OpenSim/Region/CoreModules/Asset/FlotsamAssetCache.cs

@@ -1250,6 +1250,8 @@ namespace OpenSim.Region.CoreModules.Asset
         {
             AssetBase asset;
             Get(id, out asset);
+            if (asset == null)
+                return null;
             return asset.Metadata;
         }
 
@@ -1257,6 +1259,8 @@ namespace OpenSim.Region.CoreModules.Asset
         {
             AssetBase asset;
             Get(id, out asset);
+            if (asset == null)
+                return null;
             return asset.Data;
         }