QPR-11360 check flat short end in default model

Author: hsegger

ql/experimental/credit/randomdefaultmodel.cpp

@@ -71,7 +71,7 @@ namespace QuantLib {
             Real y = a * values[0] + sqrt(1-a*a) * values[j+1];
             Real p = CumulativeNormalDistribution()(y);
 
-            if (dts->defaultProbability(tmax) < p)
+            if (dts->defaultProbability(tmax) < p || dts->hazardRate(dts->referenceDate() + 1.0 * Days ,true) < QL_EPSILON)
                 pool_->setTime(name, tmax+1);
             else
                 pool_->setTime(name, Brent().solve(Root(dts,p),accuracy_,0,1));