Use TupleDescAttr macro consistently
A few places were directly accessing the attrs[] array. This goes
against the standards set by
2cd708452. Fix that.
Discussion: https://postgr.es/m/CAApHDvrBztXP3yx=NKNmo3xwFAFhEdyPnvrDg3=M0RhDs+4vYw@mail.gmail.com