Fix use-after-free in DNS64 discovery thread am: 452f032d am: d81ab119 am: 13a28785

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/24372273



Change-Id: I141b9d48689f59d6add7897c7de23caabc16ae21
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>