This cuts out another total_num-length array and simplifies things.
Leading zeros at the front of the schedule don't do anything, so it's
easier to just produce a fixed-length one. (I'm also hoping to
ultimately reuse this function in //third_party/fiat/p256.c and get the
best of both worlds for ECDSA verification; tuned field arithmetic
operations, precomputed table, and variable-time multiply.)
Change-Id: I771f4ff7dcfdc3ee0eff8d9038d6dc9a0be3d4e0
Reviewed-on: https://boringssl-review.googlesource.com/25148
Reviewed-by: Adam Langley <agl@google.com>