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

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

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