Fix double-free in decoder_pkey.c

Fix double-free in decoder_pkey.c

Fix for the issue #13472. The decoderctx has to be initialized in every
cycle as its constructor may not be called due to lazy evaluation of
the if-condition.

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13473)