Ultra rare edge case where r == N.

1. L430 returns 0 so is NOT == 1 and if statement NOT entered.
2. L434 r.Sign() returns 1 (since N > 0) so if statement is NOT entered.
3. mod N function for s calculation is not performed until after an addition operation (if it were only Multiplication we would still arrive at 0 when mod N is performed, since a*N mod N is always 0. But there is an addition between that)
4. The Signature object is returned with R == curve.N...

Obviously I can't find a test case where nonceRFC6979 returns a k what when scalar Multiplied gets an x value == N... but N with both 0x02 and 0x03 DER headers is a valid point on the curve...

So the chance is not 0.