OpenPGP: slightly re-factor pgp_get_card_features()
OpenPGP: slightly re-factor pgp_get_card_features()

* length checks where needed
* more & better comments

viktorTarasov/OpenSC-SM,metsma/OpenSC,hhonkanen/OpenSC,dengert/OpenSC,CardContact/OpenSC,metsma/OpenSC,LudovicRousseau/OpenSC,dengert/OpenSC,CardContact/OpenSC,Jakuje/OpenSC,frankmorgner/OpenSC,fabled/OpenSC,fabled/OpenSC,mouse07410/OpenSC,LudovicRousseau/OpenSC,AktivCo/OpenSC,hongquan/OpenSC-main,rickyepoderi/OpenSC,philipWendland/OpenSC,philipWendland/OpenSC,hhonkanen/OpenSC,philipWendland/OpenSC,Jakuje/OpenSC,frankmorgner/OpenSC,frankmorgner/OpenSC,Jakuje/OpenSC,viktorTarasov/OpenSC-SM,OpenSC/OpenSC,metsma/OpenSC,viktorTarasov/OpenSC-SM,OpenSC/OpenSC,LudovicRousseau/OpenSC,rickyepoderi/OpenSC,AktivCo/OpenSC,CardContact/OpenSC,rickyepoderi/OpenSC,fabled/OpenSC,dengert/OpenSC,hongquan/OpenSC-main,frankmorgner/OpenSC,AktivCo/OpenSC,Jakuje/OpenSC,hhonkanen/OpenSC,mouse07410/OpenSC,OpenSC/OpenSC,hongquan/OpenSC-main,mouse07410/OpenSC,fabled/OpenSC