Changes

osmo_io: segmented_read: Avoid triggering read events if user unregisters

The user code doesn't expect to receive any more read cb events after
unregistering the FD. Avoid keep calling the read cb if several entire
messages were received at once in a single tcp chunk.

Related: SYS#7063
Change-Id: Id2199b9aa805cc7e7793c1a8aecd10b61f2b4c90