bn: catch negative zero as an error
bn: catch negative zero as an error

Change-Id: I5ab72ad0aae9069b47d5b7b7b9e25bd1b7afa251
Signed-off-by: Geoff Thorpe <geoff@openssl.org>

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1672)
openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl