fix the overflow/wrap checks in Samba4 for new gcc optimisation behavior

fix the overflow/wrap checks in Samba4 for new gcc optimisation behavior

The approach I have used is as set out in
https://www.securecoding.cert.org/confluence/display/seccode/ARR38-C.+Do+not+add+or+subtract+an+integer+to+a+pointer+if+the+resulting+value+does+not+refer+to+an+element+within+the+array
(This used to be commit 92d5fb531db39be655f0cbd2d75b5f675a0a4cfa)