updated fvar so if value is NaN, then derivative is set to NaN

#### Summary:
Using unlikely() compiler hint, fvar now sets the derivative to NaN if the value is NaN. 

#### Intended Effect:
Using unlikely() compiler hint, fvar now sets the derivative to NaN if the value is NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/fvar

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone.