handle the case where we try a multi-read that is completely beyond the end of file.

when a multi-read was completely beyond the end of file, this caused us to invoke the callback with a 'read-count' of <0  which the callback would treat as a failure.
This would then cause the callback to treat the data pointer as an error string and try to use it for nfs_set_error().
Since the data pointer was actually a real binary databuffer and not an error string this would cause the NFS error string to look like it contained garbage data.

In this case, where the multi-read fails to read any data at all since it if fully beyond end of file,  make sure to invoke the callback with a read-count of 0