Fix double-and-compress on Ristretto identity (issue #398).

[pornin]

This makes FieldElement::batch_invert() able to process inputs of value zero (the "inverse" of zero is formally defined to be zero; this is usually the most useful value to return in that case, and it works with the Ristretto double-and-compress formulas). I also modified the unit tests to check that functionality explicitly, both in batch_invert() itself, and in Ristretto double-and-compress.