Trouble is that addition is postponing expansion till carry is
calculated, and if addition carries, top word can be zero, which
triggers assertion in bn_check_top.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NDavid Benjamin <davidben@google.com>
(Merged from https://github.com/openssl/openssl/pull/6662)