player/lua: restore original lua allocator before close

We wrap the default Lua allocator to allow used memory tracking. This
works well, except that we never destroy the default allocator or notify
it that we are closing. Since we override it for the current Lua state,
it is reasonable that the Lua engine does not expect it to be used. We
get and pass through a free call, but in the case of LuaJIT, the
internal allocator has additional state and is freed differently. So, in
fact, it is a LuaJIT leak because once we replace the allocator with our
custom one, they should clean its internal state. I guess the assumption
is to override allocator only before any allocation happen. To work
around this issue, restore the default allocator, the one that we use,
before closing the state. This way, everything is cleared as expected.

Note that the current solution of wrapping the default allocator works
only because none of the supported Lua engines actually invalidate the
allocator on the lua_setallocf() call. However, they could, so keep in
mind that we are currently depending on an implementation detail.

Thanks to @Dudemanguy for help with finding the changes that introduced
the leak.

Fixes: a67bda2840
Fixes: #14451

player/lua.c

@@ -484,6 +484,8 @@ static int load_lua(struct mp_script_args *args)
     r = 0;
 
 error_out:
+    if (ctx->lua_allocf)
+        lua_setallocf(L, ctx->lua_allocf, ctx->lua_alloc_ud);
     if (ctx->state)
         lua_close(ctx->state);
     talloc_free(ctx);