Fix infinite recursion when using json.to on ref with cycle.

tests/stdlib/tjsonmacro.nim

@@ -354,6 +354,4 @@ when isMainModule:
     let dataDeser = to(dataParsed, Test)
     doAssert dataDeser.name == "FooBar"
     doAssert dataDeser.fallback.kind == JFloat
-    doAssert dataDeser.fallback.getFloat() == 56.42
-
-  # TODO: Cycles lead to infinite loops.
+    doAssert dataDeser.fallback.getFloat() == 56.42

tests/stdlib/tjsonmacro_reject2.nim

@@ -0,0 +1,21 @@
+discard """
+  file: "tjsonmacro_reject2.nim"
+  line: 10
+  errormsg: "The `to` macro does not support ref objects with cycles."
+"""
+import json
+
+type
+  Misdirection = object
+    cycle: Cycle
+
+  Cycle = ref object
+    foo: string
+    cycle: Misdirection
+
+let data = """
+  {"cycle": null}
+"""
+
+let dataParsed = parseJson(data)
+let dataDeser = to(dataParsed, Cycle)