Check return value of some BN functions.

Check return value of some BN functions.

Factorise multiple bn_get_top(group->field) calls
Add missing checks on some conditional BN_copy return value
Add missing checks on some BN_copy return value
Add missing checks on a few bn_wexpand return value

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1626)
(cherry picked from commit 78e09b53a40729f5e99829ccc733b592bd22fea1)