DECODER: check the first decoded structure name against user given structure

DECODER: check the first decoded structure name against user given structure

In a chain of decoders, the first that specifies an input structure
gets it compared with the structure specified by the user, if there is
one.  If they aren't the same, that decoder is skipped.

Because the first structure can appear anywhere along a chain of
decoders, not just the decoders associated with the resulting OpenSSL
type, the code that checked the structure name when building up the
chain of decoders is removed.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16466)