nfs4: Remove assertion against NFS4ERR_NOFILEHANDLE

According to the NFS4 specification NFS4ERR_NOFILEHANDLE is returned only
when the clients sends malformed request. FreeBSD nfsd implementation chooses
to ignore that fact and returns this error code also for correctly formed
requests that it can not service due to the restrictions in the server
configuration.
nfs4: Fix double free when NFS4Inode::ReadDirOnce fails