cups with current lspp patch returnes HTTP_STATUS_SERVER_ERROR (internal
server error) if user has selinux context != UNKNOWN_SL and ipp call
doesn't contain a file.

--- cups/scheduler/client.c
+++ cups/scheduler/client.c
@@ -2008,7 +2008,7 @@ cupsdReadClient(cupsd_client_t *con)	/* I - Client to read from */
 	  }
 
 #ifdef WITH_LSPP
-	  if (strncmp(con->scon, UNKNOWN_SL, strlen(UNKNOWN_SL)) != 0)
+	  if (con->filename && strncmp(con->scon, UNKNOWN_SL, strlen(UNKNOWN_SL)) != 0)
 	  {
 	    if (getfilecon(con->filename, &spoolcon) == -1)
 	    {