Fix use-after-free in DNS64 discovery thread am: 452f032d am: 5be90308

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



Change-Id: I15b02321d0ef34d64819dc1d502ca270b9ca9a22
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>