GFS: Avoid piggyback for hits < 0 from backend
GFS: Avoid piggyback for hits < 0 from backend

If a backend returns negative hit count, then avoid comparisons against
smallSetUpperBound and largeSetLowerBound. Otherwise a negative size
in call to odr_malloc will occur.

dcrossleyau/yaz,dcrossleyau/yaz,nla/yaz,nla/yaz,nla/yaz,dcrossleyau/yaz,nla/yaz