BUG/MINOR: ssl: Missing calloc return value check in ssl_init_single_engine

BUG/MINOR: ssl: Missing calloc return value check in ssl_init_single_engine

A memory allocation failure happening during ssl_init_single_engine
would have resulted in a crash. This function is only called during
init.

It was raised in GitHub issue #1233.
It could be backported to all stable branches.

(cherry picked from commit 612b2c37be70636e73698c6c064af9522301be65)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 46697c80fec88e656a4f7280a2cfa9dedbfb33a5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>