BUG/MINOR: lua: Add missing string length for lua sticktable lookup

In hlua_stktable_lookup(), the key length is never set so all
stktable:lookup("key") calls return nil from lua.

This patch must be backported as far as 1.9.

[Cf: I slightly updated the patch to use lua_tolstring() instead of
     luaL_checkstring() + strlen()]
1 file changed