BUG/MINOR: ssl: init tmp chunk correctly in ssl_sock_load_sctl_from_file()

BUG/MINOR: ssl: init tmp chunk correctly in ssl_sock_load_sctl_from_file()

Use chunk_inistr() for a chunk initialisation in
ssl_sock_load_sctl_from_file() instead of a manual initialisation which
was not initialising head.

Fix issue #1073.

Must be backported as far as 2.2

(cherry picked from commit 8d67394f6915c6d2db40bc1e9593fd392827da8d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 7cdbb5643de32c97f9d7693f140b89f4a4cb9493)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 5df6c0815064a48111d6a35b9ce29677e087a0b0)
[cf: Backport to 2.1 also required; context adjustment]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>