OpenSSL: fix erroneous SSL backend encapsulation

In d65e6cc4 (vtls: prepare the SSL backends for encapsulated private
data, 2017-06-21), this developer prepared for a separation of the
private data of the SSL backends from the general connection data.

This conversion was partially automated (search-and-replace) and
partially manual (e.g. proxy_ssl's backend data).

Sadly, there was a crucial error in the manual part, where the wrong
handle was used: rather than connecting ssl[sockindex]' BIO to the
proxy_ssl[sockindex]', we reconnected proxy_ssl[sockindex]. The reason
was an incorrect location to paste "BACKEND->"... d'oh.

Reported by Jay Satiro in https://github.com/curl/curl/issues/1855

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