Iterate over EC_GROUP's poly array in a safe way

Prevent that memory beyond the last element is accessed if every element
of group->poly[] is non-zero

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2689)

(cherry picked from commit 57f48f93)