Merge #754: Fix uninit values passed into cmov

Merge #754: Fix uninit values passed into cmov

f79a7adcf555ccc78b591850ea15c64fbfbca152 Add valgrind uninit check to cmovs output (Elichai Turkel)
a39c2b09de304b8f24716b59219ae37c2538c242 Fixed UB(arithmetics on uninit values) in cmovs (Elichai Turkel)

Pull request description:

  This should fix #753.
  Used @peterdettman's solution here for the `ECMULT_CONST_TABLE_GET_GE` https://github.com/bitcoin-core/secp256k1/issues/753#issuecomment-631316091
  and in ecdsa_sign I initialize `s` and `r` to a zero scalar.

  The second commit adds a valgrind check to the cmovs that could've caught this (in ecdsa_sign, not in ecmult_const because there's a scalar clear there under `VERIFY_SETUP`)

ACKs for top commit:
  sipa:
    utACK f79a7adcf555ccc78b591850ea15c64fbfbca152
  jonasnick:
    ACK f79a7adcf555ccc78b591850ea15c64fbfbca152
  real-or-random:
    ACK f79a7adcf555ccc78b591850ea15c64fbfbca152

Tree-SHA512: 6fd7b7c84f392bda733a973f4dcfc12bf1478aac2591e2c87b69e637847d3b063c4243cc8feccaffc3a5824c18183a5e66bd4251c2322abaf63bb6439b38defe