BUG/MINOR: hlua: fix uninitialized var in hlua_core_get_var()

BUG/MINOR: hlua: fix uninitialized var in hlua_core_get_var()

As raised by Coverity in GH #2223, f034139bc0 ("MINOR: lua: Allow reading
"proc." scoped vars from LUA core.") causes uninitialized reads due to
smp being passed to vars_get_by_name() without being initialized first.

Indeed, vars_get_by_name() tries to read smp->sess and smp->strm pointers.
As we're only interested in the PROC var scope, it is safe to call
vars_get_by_name() with sess and strm pointers set to NULL, thus we
simply memset smp prior to calling vars_get_by_name() to fix the issue.

This should be backported in 2.9 with f034139bc0.