* fix: remove deadlock in `multiPacketListener`.

* fix test error

* Move reading from the `PacketConn` outside the `select`.

* Revert "fix test error"

This reverts commit 552a1e60a44470207ffed7d86edb2e59032b5be8.

* Do not set `m.pc` to nil.

* Move buffer down.

* Undo vpc creation.

* Add mutex back to protect against concurrent `Close()` calls.

* Move return when connection is closed down.

* Do not close the `readCh` or exit on a closed PacketConn.