Fix: consumer_allocate_stream error handling

Fix a memory leak and "be nice" when handling stream alloc errors. Upon
CPU hotplug, it is possible that we receive a stream only after all
other streams are finalized, which means it could happen that we discard
that channel, in the unlikely event that we have cpu hotplug
concurrently with destroy.

Moreover, this fix the return path of channel lookup failure: we were
returning an zeroed stream rather than returning an error, which was
certainly not the intended behavior.

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