Fix use-after-free when forcibly reloading the luv module

[squeek502]

Closes #819

I was still not able to reproduce the bug with garbage collection alone, but this also fixes a modified version of the reproduction with an explicit loop_close:

local uv = require "luv"

local timer = uv.new_timer()
timer:start(100, 0, function() timer:close() end)
uv.run()

package.loaded["luv"] = nil
-- forcibly close the loop
uv.loop_close()

-- Step 3: Re-require and use → SEGFAULT (previously)
local uv2 = require "luv"
local timer2 = uv2.new_timer()  -- no segfault here :)