Harmonize use of sk_TYPE_find's return value.

In some cases it's about redundant check for return value, in some
cases it's about replacing check for -1 with comparison to 0.
Otherwise compiler might generate redundant check for <-1. [Even
formatting and readability fixes.]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6860)