Simplify code around next_proto.len by changing 'len' data type.

clean an useless static qualifier and a dead comment.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2278)

(cherry picked from commit f2ff1432)