nfscl: Check for an error return from nfsrv_getattrbits()

There were two places where the client code did not check
for a parse error return from nfsrv_getattrbits().

This patch fixes both of these cases.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260272
MFC after: 2 weeks