lang/lua53: move assertion to correct place

[russ.haley_gmail.com]

Summary

This patches an issue when the assert option is enabled, which is one of the build options in the port. The patch was provided by @andrew_tao173.riddles.org.uk.

Reference: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236891

Test Plan

I have no way to test mitigation provided by the patch. However the port build clean when the assert mode option is enabled. Any test suggestions are welcome

[andrew_tao173.riddles.org.uk]

The test case I've been using was posted to lua-l in Feb by Sascha Zelzer, but the archives aren't public so I can't just link to it.

Here it is (though I've increased the iteration count from the original - it's worth playing with that a bit if you can't get it to easily fail). It doesn't always fail, since the window of vulnerability is quite narrow (it fails if a closure is closed over a value-weak or ephemeron (key-weak) table reference in between the final mark/propagation step of a GC cycle and the following atomic step), but I could get occasional assertion failures with this on the old code, and not on the patched version.

local function f()
  local weaktable = setmetatable({}, { __mode = "v" })
 
  return function()
    local row = weaktable[1]
    if not row then
      row = { a = 1 }
      weaktable[1] = row
    end
    return row
  end
end

for i=1,100000000 do
  local t = f()()
end

[russ.haley_gmail.com]

This has been implemented as a ports option and is already in the tree