BUG/MEDIUM: ssl: Spell HAVE_VANILLA_OPENSSL correctly

Use HAVE_VANILLA_OPENSSL, which is what we actually define when we're
linked against OpenSSL, and not USE_VANILLA_OPENSSL.
Not using the right macro means we may start splicing with kTLS while
there are still data available in OpenSSL's internal buffers, leading to
data not being properly read.

This should be backported up to 3.3.