Fix crash when bond tenor length is 0

In the constructor of ```AmortizingFixedRateBond```, if ```bondTenor``` is accidentally set as length 0, the program will crash from the line 124 in file ```amortizingfixedratebond.cpp```. ```nPeriods``` will be start as 0 and the statement ```(Size)nPeriods-1``` will roll to a very large positive number.