sync::mpsc: synchronize receiver disconnect with initialization
Receiver disconnection relies on the incorrect assumption that `head.index != tail.index` implies that the channel is initialized (i.e `head.block` and `tail.block` point to allocated blocks). However, it can happen that `head.index != tail.index` and `head.block == null` at the same time which leads to a segfault when a channel is dropped in that state.
This can happen because initialization is performed in two steps. First, the tail block is allocated and the `tail.block` is set. If that is successful `head.block` is set to the same pointer. Importantly, initialization is skipped if `tail.block` is not null.
Therefore we can have the following situation:
1. Thread A starts to send the first value of the channel, observes that `tail.block` is null and begins initialization. It sets `tail.block` to point to a newly allocated block and then gets preempted. `head.block` is still null at this point.
2. Thread B starts to send the second value of the channel, observes that `tail.block` *is not* null and proceeds with writing its value in the allocated tail block and sets `tail.index` to 1.
3. Thread B drops the receiver of the channel which observes that `head.index != tail.index` (0 and 1 respectively), therefore there must be messages to drop. It starts traversing the linked list from `head.block` which is still a null pointer, leading to a segfault.
This PR fixes this problem by waiting for initialization to complete when `head.index != tail.index` and the `head.block` is still null. A similar check exists in `start_recv` for similar reasons.
Fixes#110001
f56a265105