The control block length rule in Script Validation caps m at 128, but
compute_merkle_root and compute_control_block applied no depth bound, so a
129-deep tree produced a 4129-byte control block for an output consensus
must reject as unspendable.
Structural validation also used assert, which python -O strips: a malformed
3-child branch silently returned the root of its first two leaves rather
than raising, dropping a script leaf from the commitment.
Adds negative_structure_tests covering both, run from BIP360_tests.
