fix offw computation in readPtr() for negative val offsets (val<0 does happen, was causing data loss before fix).
fix offw computation in readPtr() for negative val offsets (val<0 does happen, was causing data loss before fix).

zombiezen/go-capnproto2,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto,tpukep/caps,hodduc/go-capnproto2