BUG/MINOR: hlua: fix format-string vulnerability in Patref error path

BUG/MINOR: hlua: fix format-string vulnerability in Patref error path

hlua_error() is a printf-family function (calls vsnprintf), but
hlua_patref_set, hlua_patref_add, and _hlua_patref_add_bulk pass
errmsg directly as the format string. errmsg is built by pattern.c
helpers that embed the user-supplied key or value verbatim, e.g.
pat_ref_set_elt() generates "unable to parse '<value>'".

A Lua script calling:

    ref:set("key", "%p.%p.%p.%p.%p.%p.%p.%p")

against a map with an integer output type (where the parse fails)
gets stack/register contents formatted into the (nil, err) return
value -> ASLR/canary leak. With %n and no _FORTIFY_SOURCE this
becomes an arbitrary write primitive.

This must be backported as far as the Patref Lua API exists.