Merge #592: Use trivial algorithm in ecmult_multi if scratch space is small

Merge #592: Use trivial algorithm in ecmult_multi if scratch space is small

9ab96f7 Use trivial algorithm in ecmult_multi if scratch space is small (Jonas Nick)

Pull request description:

  `ecmult_multi` already selects the trivial algorithm if the scratch space is NULL. With this PR the trivial algorithm is also selected if the scratch space is too small to use pippenger or strauss instead of returning 0. That makes it more easier to avoid consensus relevant inconsistencies just because scratch space construction was messed up.

ACKs for commit 9ab96f:
  real-or-random:
    utACK 9ab96f7

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