ec/ec_mult.c: get BN_CTX_start,end sequence right.

Triggered by Coverity analysis.

Reviewed-by: Rich Salz <rsalz@openssl.org>

(cherry picked from commit 7d859d1c

)

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6549)