The depth field was never decremented inside w_object(), and it was
never initialized in PyMarshal_WriteObjectToFile().

This caused imports from .pyc files to fil mysteriously when the .pyc
file was written by the broken code -- w_object() would bail out
early, but PyMarshal_WriteObjectToFile() doesn't check the error or
return an error code, and apparently the marshalling code doesn't call
PyErr_Check() either.  (That's a separate patch if I feel like it.)