BUG: special/specfun: fix argument value check in KLVNZO

documentation of KLVNZO(NT,KD,ZO) in scipy/special/specfun.f
says 1<=KD<=8. Previous code resulted in always True.
Merge pull request #2780 from aleufroy/fix-2718

BUG: special/specfun: fix argument value check in KLVNZO