Don't try the same decoder multiple times

Don't try the same decoder multiple times

The function collect_decoder decides whether a given decoder should be
tried or not. It loops through all the names for matching keymgmts to
see if any are a match or not. If there is a match then the decoder gets
added. However, each keymgmt may have multiple aliases and a decoder was
being added for each one. For example DHX has 4 alias names, and therefore
4 instances of the DHX decoder were added and being tried.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15404)