Fix: Sync issue when deleting a data stream

A data stream was rescheduled for deletion after a flush on hang up.
Once in a normal read code path, on error, the stream is deleted and
then processed for ERR|HUP error which could also delete again the
stream causing an assert() and a failed trace.

We fix that by setting the local array to NULL for that stream once
deleted and ignoring the stream is subsequent loop if NULL.

Fixes #390

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>