WPACKET: don't write DER length when we don't want to

WPACKET: don't write DER length when we don't want to

With endfirst writing, it could be that we want to abandon any zero
length sub-packet.  That's what WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH
was supposed to make happen, but the DER length writing code didn't
look at that flag.  Now it does.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/11703)