nfsd: more careful input validation in nfsctl write methods

Neil Brown points out that we're checking buf[size-1] in a couple places
without first checking whether size is zero.

Actually, given the implementation of simple_transaction_get(), buf[-1]
is zero, so in both of these cases the subsequent check of the value of
buf[size-1] will catch this case.

But it seems fragile to depend on that, so add explicit checks for this
case.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: NeilBrown <neilb@suse.de>