fix an off-by-one error in number conversion error checking
fix an off-by-one error in number conversion error checking

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,perl6/nqp-rx,perl6/nqp-rx,cygx/nqp,cygx/nqp,cygx/nqp