multiplex: fix crash when second concurrent client disconnects
multiplex: fix crash when second concurrent client disconnects

Add a missing assignment for the previous client, such that when we
remove a client in the middle of the chain, we actually have the pointer
to the previous.  Fixes a crash when the second (or higher) client
disconnects while the first client remains connected.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb