OpenSSL: fix yet another mistake while encapsulating SSL backend data

Another mistake in my manual fixups of the largely mechanical
search-and-replace ("connssl->" -> "BACKEND->"), just like the previous
commit concerning HTTPS proxies (and hence not caught during my
earlier testing).

Fixes #1855
Closes #1871

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>