MGRS.cpp: fix checs on zone and prec.
MGRS.cpp: fix checs on zone and prec.

Sciumo/GeographicLib,Sciumo/GeographicLib,geographiclib/geographiclib,Sciumo/GeographicLib,geographiclib/geographiclib,Sciumo/GeographicLib,geographiclib/geographiclib,geographiclib/geographiclib,Sciumo/GeographicLib,Sciumo/GeographicLib,Sciumo/GeographicLib,Sciumo/GeographicLib