scheduler: Fix regression in background thread (Issue #934)

The con->bg_pending value is used to indicate that the background thread
has finished running and that the connection can be safely closed.

This commit makes the con->bg_pending = 0 only after the connection
can be safely closed.

Author: andreas-karlson

scheduler/ipp.c

@@ -5517,10 +5517,10 @@ create_local_bg_thread(
 
   ippDelete(response);
 
-  con->bg_pending = 0;
-
   send_response(con);
 
+  con->bg_pending = 0;
+
   return (NULL);
 }