Fix GCF extraction (#48)

Author: HeroponRikiBestest

SabreTools.Serialization/Readers/GCF.cs

@@ -304,7 +304,7 @@ public class GCF : BaseBinaryReader<Data.Models.GCF.File>
                 #endregion
 
                 // Seek to end of checksum section, just in case
-                data.SeekIfPossible(afterMapPosition + checksumHeader.ChecksumSize, SeekOrigin.Begin);
+                data.SeekIfPossible(initialOffset + checksumHeader.ChecksumSize, SeekOrigin.Begin);
 
                 #region Data Block Header