avoid bad error check of slicecred when cred is struct
avoid bad error check of slicecred when cred is struct

GENI-NSF/gram,tcmitchell/geni-tools,GENI-NSF/gram,ahelsing/geni-tools,GENI-NSF/gram,ahelsing/geni-tools,plantigrade/geni-tools,plantigrade/geni-tools,tcmitchell/geni-tools