Use trivial algorithm in ecmult_multi if scratch space is small

Summary: This is a backport of secp256k1 PR592

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5032