Add extra validity checks for GCF (#17)

* Add extra validity checks for GCF

* Remove "for validity"

Author: HeroponRikiBestest

SabreTools.Serialization/Deserializers/GCF.cs

@@ -104,8 +104,15 @@ public class GCF : BaseBinaryDeserializer<Models.GCF.File>
 
                 #region Directory Header
 
+                // Try to parse game cache directory header
+                var directoryHeader = ParseDirectoryHeader(data);
+                if (directoryHeader.Dummy0 != 0x00000004)
+                    return null;
+                if (directoryHeader.Dummy1 != 0x00008000)
+                    return null;
+                
                 // Set the game cache directory header
-                file.DirectoryHeader = ParseDirectoryHeader(data);
+                file.DirectoryHeader = directoryHeader;
 
                 #endregion
 
@@ -662,4 +669,4 @@ public static Header ParseHeader(Stream data)
             return obj;
         }
     }
-}
+}