bn/bn_lib.c: remove bn_check_top from bn_expand2.

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: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6810)

(cherry picked from commit e42395e6)

Resolved conflicts:
	crypto/bn/bn_lib.c