BUG/MINOR: hlua: fix leak in hlua_ckch_set() error path

BUG/MINOR: hlua: fix leak in hlua_ckch_set() error path

in hlua_ckch_commit_yield() and hlua_ckch_set(), when an error occurs,
we enter the error path and try to raise an error from the <err> msg
pointer which must be freed afterwards.

However, the fact that luaL_error() never returns was overlooked, because
of that <err> msg is never freed in such case.

To fix the issue, let's use hlua_pushfstring_safe() helper to push the
err on the lua stack and then free it before throwing the error using
lua_error().

It should be backported up to 2.6 with 30fcca18 ("MINOR: ssl/lua:
CertCache.set() allows to update an SSL certificate file")