Fix use-after-free in DNS64 discovery thread am: 292b59e1 am: 93d3a164

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/DnsResolver/+/24557576



Change-Id: Ibf138a584cd17293e5776fadccb2722c27f541c8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>