Initializes greeks and cheks vanilla values in fwd evaluation

Greeks and MoreGreeks aren't initialized on construction, leading to errors when using quanto and forward engines.
Besides, forward engine doesn't check embedded engine results, returning bogus values when the vanilla engine can't calculate some greeks.
All tests passing.
Is this necessary, given the call to `originalEngine_->reset()` below? 
Of course it doesn't hurt, but it seems unnecessary if the data members will be overwritten by that call anyway.