Fix IndexOutOfRangeException

Fixes situations where the filename is something like "-------".

MediaBrowser.Server.Implementations/Library/LibraryManager.cs

@@ -923,14 +923,14 @@ namespace MediaBrowser.Server.Implementations.Library
 
             if (type == typeof(Person))
             {
-                var subFolderIndex = 0;
+                foreach (char c in validFilename)
-
-                while (!char.IsLetterOrDigit(validFilename[subFolderIndex]))
                 {
-                    subFolderIndex++;
+                    if (char.IsLetterOrDigit(c))
+                    {
+                        subFolderPrefix = c.ToString();
+                        break;
+                    }
                 }
-
-                subFolderPrefix = validFilename.Substring(subFolderIndex, 1);
             }
 
             var fullPath = string.IsNullOrEmpty(subFolderPrefix) ?