Fix BN_DEBUG: ossl_assert() → assert()

Fix BN_DEBUG: ossl_assert() → assert()

ossl_assert() has been modified so much that it no longer fits the
purpose of bn_check_top() when BN_DEBUG is defined in a debug build,
which is to abort and tell where the BIGNUM is inconsistent.  This
is by design.

This has remained undiscovered because no one has tried BN_DEBUG
for quite a while.

Assertions in bn_check_top() are also rearranged to better show what
the actual problem is.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28801)