upspin: fix bug in SortDirEntries
upspin: fix bug in SortDirEntries

"result" may be empty; look at previous entry in slice.

Change-Id: Ifbaf7a976f116c87e19d1730d803d97f7e556f0a
Reviewed-on: https://upspin-review.googlesource.com/4723
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin