Protect Tcl_GetIndexFromObjStruct from invalid "offset" values, like 0 or -1. Undocumented, because I don't want to promote people start using that.
Simplify STRING_AT macro.
Protect Tcl_GetIndexFromObjStruct from invalid "offset" values, like 0 or -1. Undocumented, because I don't want to promote people start using that.