vtls: fix memory corruption

Ever since 70f1db32 (vtls: encapsulate SSL backend-specific data,
2017-07-28), the code handling HTTPS proxies was broken because the
pointer to the SSL backend data was not swapped between
conn->ssl[sockindex] and conn->proxy_ssl[sockindex] as intended, but
instead set to NULL (causing segmentation faults).

[jes: provided the commit message, tested and verified the patch]

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