Mark pop/clear error stack in der2key_decode_p8

Mark pop/clear error stack in der2key_decode_p8

This commit sets the error mark before calling d2i_X509_SIG
and clear it if that function call is successful.

The motivation for this is that if d2i_X509_SIG returns NULL then the
else clause will be entered and d2i_PKCS8_PRIV_KEY_INFO will be
called. If d2i_X509_SIG raised any errors those error will be on the
error stack when d2i_PKCS8_PRIV_KEY_INFO gets called, and even if it
returns successfully those errors will still be on the error stack.

We ran into this issue when upgrading Node.js to 3.0.0-alpha15.
More details can be found in the ref links below.

Refs: https://github.com/nodejs/node/issues/38373
Refs: https://github.com/danbev/learning-libcrypto/blob/master/notes/wrong-tag-issue2.md

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/15067)