BUG/MINOR: lua: initialize sample before using it

BUG/MINOR: lua: initialize sample before using it

Memset the sample before using it through hlua_lua2smp. This function is
ORing the smp.flags, so this field need to be cleared before its use.
This was reported by a coverity warning.

Fixes the github issue #929.
This bug can be backported up to 1.8.

(cherry picked from commit bc0af6a19947c7ab76bd226beaaaafedc0648be4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 42cd304165f9afa2050f169e7c852f2f637bcc70)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>