Fix __gc and use raw metatable when it's disabled

This fixes a small __gc abuse I found, and also uses the raw metatable in a safe way when __gc is disabled. Both were tested.
# Conflicts:
#	src/main/resources/assets/opencomputers/lua/machine.lua

src/main/resources/assets/opencomputers/lua/machine.lua

@@ -683,7 +683,7 @@ sandbox = {
     end
     local result = getmetatable(t)
     -- check if we have a wrapped __gc using mt
-    if type(result) == "table" and rawget(result, "__gc") == sgc then
+    if type(result) == "table" and system.allowGC() and rawget(result, "__gc") == sgc then
       result = rawget(result, "mt")
     end
     return result
@@ -713,7 +713,11 @@ sandbox = {
     if type(mt) ~= "table" then
       return setmetatable(t, mt)
     end
-    if type(rawget(mt, "__gc")) == "function" then
+    if rawget(mt, "__gc") ~= nil then -- If __gc is set to ANYTHING not `nil`, we're gonna have issues
+      -- Garbage collector callbacks apparently can't be sandboxed after
+      -- all, because hooks are disabled while they're running. So we just
+      -- disable them altogether by default.
+      if system.allowGC() then
       -- For all user __gc functions we enforce a much tighter deadline.
       -- This is because these functions may be called from the main
       -- thread under certain circumstanced (such as when saving the world),
@@ -723,16 +727,18 @@ sandbox = {
       for k, v in pairs(mt) do
         sbmt[k] = v
       end
+        sbmt.__gc = sgc
       sbmt.mt = mt
-      -- Garbage collector callbacks apparently can't be sandboxed after
-      -- all, because hooks are disabled while they're running. So we just
-      -- disable them altogether by default.
-      if not system.allowGC() then
-        sbmt.__gc = nil -- Silent fail for backwards compat. TODO error in OC 1.7
+        mt = sbmt
       else
-        sbmt.__gc = sgc
+        -- Don't allow marking for finalization, but use the raw metatable.
+        local gc = rawget(mt, "__gc")
+        rawset(mt, "__gc", nil) -- remove __gc
+        local ret = table.pack(pcall(setmetatable, t, mt))
+        rawset(mt, "__gc", gc) -- restore __gc
+        if not ret[1] then error(ret[2], 0) end
+        return table.unpack(ret, 1, ret.n)
       end
-      mt = sbmt
     end
     return setmetatable(t, mt)
   end,
@@ -1442,4 +1448,4 @@ end
 
 -- JNLua converts the coroutine to a string immediately, so we can't get the
 -- traceback later. Because of that we have to do the error handling here.
-return pcall(main)
+return pcall(main)