fix nim-lang#18971 (nim-lang#19070) [backport:1.6]

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

compiler/varpartitions.nim

@@ -479,7 +479,7 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
         # calls do construct, what we construct must be destroyed,
         # so dest cannot be a cursor:
         dest.flags.incl ownsData
-      elif n.typ.kind in {tyLent, tyVar}:
+      elif n.typ.kind in {tyLent, tyVar} and n.len > 1:
         # we know the result is derived from the first argument:
         var roots: seq[(PSym, int)]
         allRoots(n[1], roots, RootEscapes)

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()