Bluetooth: Audio: Add missing NULL checks when looking up audio_iso streams

Multiple places we assumed that we could simply just do
sink_stream->iso == chan without checking if sink_stream
was NULL.

This commit fixes that by adding proper NULL checks for that,
as well as additional checks before defaulting to the
source_stream.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>