Infinite iteration in _AddListFieldsMethod when serializing nested messages

[mrgonza78]

// issue.proto
syntax = "proto2";

message Qux {
  optional uint32 waldo = 1;
}

message Bar {
  optional Qux qux = 1;
}

message Foo {
  optional Bar bar = 1;
}

issue_pb = require('issue_pb')
foo = issue_pb.Foo()
foo.bar.qux.waldo = 1
foo:SerializeToString()

SerializeToString loops infinitely at _AddListFieldsMethod.

The problem seems to be that bar. _is_present_in_parent is false b/c bar.SetInParent is never called (may be?)

If I manually call bar.SetInParent, then it just works

issue_pb = require('issue_pb')
foo = issue_pb.Foo()
foo.bar.qux.waldo = 1
bar:SetInParent()
foo:SerializeToString() -- Now it works, I get the string