fix #18971

since the example code return value from global variable, instead
of first argument, the `n.len` is 1 which causes compiler crashes.

tests/arc/t18971.nim

@@ -0,0 +1,10 @@
+discard """
+  cmd: "nim c --gc:arc $file"
+"""
+
+type MyObj = ref object
+
+var o = MyObj()
+proc x: var MyObj = o
+
+var o2 = x()