Length fields should default to zero (#22)

This fixes issue (#22). We choose not to remove the field as it's in the specs and migth be used in some archive files (while never saw it actually).

pysap/SAPCAR.py

@@ -188,8 +188,8 @@ class SAPCARArchiveFilev200Format(PacketNoPadded):
         LEIntField("file_length_high", 0),
         LELongField("timestamp", 0),
         LEIntField("code_page", 0),
-        FieldLenField("user_info_length", None, length_of="user_info", fmt="<H"),
-        FieldLenField("filename_length", None, length_of="filename", fmt="<H"),
+        FieldLenField("user_info_length", 0, length_of="user_info", fmt="<H"),
+        FieldLenField("filename_length", 0, length_of="filename", fmt="<H"),
         StrNullFixedLenField("filename", None, length_from=lambda x: x.filename_length,
                              null_terminated=lambda x: x.is_filename_null_terminated),
         StrFixedLenField("user_info", None, length_from=lambda x: x.user_info_length),