PKCS#7: Fix the parser cleanup to drain parsed out X.509 certs

Fix the parser cleanup code to drain parsed out X.509 certs in the case that
the decode fails and we jump to error_decode.

The function is rearranged so that the same cleanup code is used in the success
case as the error case - just that the message descriptor under construction is
only released if it is still pointed to by the context struct at that point.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>