Fix for #101 NullArgumentException in AzureFileSystem.FixPath

Author: Jeavon

src/UmbracoFileSystemProviders.Azure/AzureFileSystem.cs

@@ -695,7 +695,7 @@ private string FixPath(string path)
             }
 
             string appVirtualPath = this.ApplicationVirtualPath;
-            if (path.StartsWith(appVirtualPath))
+            if (appVirtualPath != null && path.StartsWith(appVirtualPath))
             {
                 path = path.Substring(appVirtualPath.Length);
             }