repmgrd: ensure witness node doesn't try and follow another witness

Theoretically there should never be more than one witness node
visible here, but it's not impossible to rule it out, so add a
check just in case.

repmgrd-physical.c

@@ -1467,6 +1467,12 @@ monitor_streaming_witness(void)
 							continue;
 						}
 
+						/* skip node if configured as a witness node - we can't possibly "follow" that */
+						if (cell->node_info->type == WITNESS)
+						{
+							continue;
+						}
+
 						cell->node_info->conn = establish_db_connection(cell->node_info->conninfo, false);
 
 						if (PQstatus(cell->node_info->conn) != CONNECTION_OK)