scheduler: Fix potential bug when adding new printers (Issue #934)

When adding a new printer a thread is created to handle PPD generation,
and the requesting client connection might close while the thread is running.
This commit prevents full connection cleanup until the thread is finished.

Author: andreas-karlson

scheduler/client.c

@@ -430,6 +430,14 @@ cupsdCloseClient(cupsd_client_t *con)	/* I - Client to close */
     con->file = -1;
   }
 
+  if (con->bg_pending)
+  {
+   /*
+    * Don't close connection when there is a background thread pending
+    */
+    partial = 1;
+  }
+
  /*
   * Close the socket and clear the file from the input set for select()...
   */