Fix #14160 (#14161)

* Fix #14160

* Add testcase

tests/ccgbugs/t14160.nim

@@ -0,0 +1,15 @@
+type
+  TPassContext = object of RootObj
+  PPassContext = ref TPassContext
+
+  PCtx = ref object of TPassContext
+    a: int
+
+  ModuleGraph = object
+    vm: RootRef
+
+proc main() =
+  var g = ModuleGraph(vm: new(Pctx))
+  PCtx(g.vm) = nil #This generates invalid C code
+
+main()