Prevent ints from wrapping around in scratch space functions

Summary:

• Add check preventing integer multiplication wrapping around in scratch_max_allocation
• Add check preventing rounding to alignment from wrapping around in scratch_alloc
• Use ROUND_TO_ALIGN in scratch_create
• Don't assume that ALIGNMENT > 1 in tests

This is a backport of secp256k1 PR648

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7608