Co-authored-by: nullun <git@nullun.com>

Pavel/skip block assembly crash

The per-call-site "if len(b) == 0 { NULL } else { &b[0] }" branches
look like they should be a shared helper, but the duplication is
load-bearing: cgo enforces its pointer-passing rules against the
syntactic form of the call arguments, so an inline &b[0] is checked
against just the slice's backing array, while the same pointer
returned from a helper is resolved to its entire containing
allocation -- and panics when the caller's slice aliases a struct
that also holds Go pointers, exactly what TestPointerToPointer and
TestDet512PointerToPointer construct. Say so where the next reader
would reach for the helper.

falcon_det{1024,512}_pubkey_coeffs skipped the first byte of the key and
decoded the remaining bytes without ever looking at it, so a buffer whose
header byte is wrong (a key for a different logn, or bytes that are not a
Falcon public key at all) was unpacked as though it were valid, returning 0
and a coefficient vector that does not correspond to any det key. The
verify entry points do not have this gap: they go through falcon_verify,
which checks the header before touching the rest of the key. Check the
byte here the same way and return FALCON_ERR_FORMAT, which the header's
"non-zero error code if pubkey is invalid" contract already promises.

deterministic1024.o and deterministic512.o were in the shared OBJ list, so
every binary linked both, including test_falcon and speed, which use
neither, and each KAT driver, which uses only its own variant. Give each
test_deterministic<n> binary its own deterministic<n>.o prerequisite and
keep the generic objects in OBJ, so a binary pulls in only the variant it
exercises. The objects still have their own rules and "clean" still
removes them.