Merge #732: Retry if r is zero during signing

37ed51a7eafa5fd9955ceb66213251cdd8b8b8c4 Make ecdsa_sig_sign constant-time again after reverting 25e3cfb (Tim Ruffing)
93d343bfc5323e56f6a60cb41d60b96368cc09c7 Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ecdsa_sig_sign" (Tim Ruffing)

Pull request description:

ACKs for top commit:
  elichai:
    ACK 37ed51a7eafa5fd9955ceb66213251cdd8b8b8c4 makes sense.
  jonasnick:
    ACK 37ed51a7eafa5fd9955ceb66213251cdd8b8b8c4

Tree-SHA512: 82b5b8e29f48e84fd7a0681b62923d3bd87d724b38ef18e8c7969b0dcc5a405ebb26c14b5c5f4c7ba0ccabd152d1531d217809d1daf40872fe0c1e079b55c64b