Windows: Prevent fs fetchdata offset bug error from propagating to caller

Windows: Prevent fs fetchdata offset bug error from propagating to caller

When the file server fetchdata offset bug is triggered and detected,
not only do we have to behave as if 'length_found' is zero, but we
also must ignore the RXGEN_CC_UNMARSHAL error that will be returned
from rx_EndCall() due to the parsing error that occurred during the
rx_Read32() call.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/634
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>