fix off-by-one error in String.cstring (#2616)
fix off-by-one error in String.cstring (#2616)

that could surface if the newly allocated cstring buffer is not set ot 0 completely
sgebbie/ponyc,dipinhora/ponyc,ponylang/ponyc,cquinn/ponyc,dipinhora/ponyc,kulibali/ponyc,cquinn/ponyc,CausalityLtd/ponyc,kulibali/ponyc,Theodus/ponyc,Theodus/ponyc,cquinn/ponyc,kulibali/ponyc,jemc/ponyc,ponylang/ponyc,sgebbie/ponyc,CausalityLtd/ponyc,Theodus/ponyc,sgebbie/ponyc,ponylang/ponyc,sgebbie/ponyc,sgebbie/ponyc,jemc/ponyc,kulibali/ponyc,Theodus/ponyc,cquinn/ponyc,dipinhora/ponyc,jemc/ponyc,Theodus/ponyc,CausalityLtd/ponyc