process: avoid negative number of children

The number_of_children child connections number becomes negative (-1) at
server startup due to signal handlers treatments on minidlna.c init().

Opening the webserver status page with no clients connected the opening
connection shows -1 even if client list show 0 for all in the columns.

When connecting the first client, its own connections column goes to 1
and total goes to 0, and so on always one count behind.

Simply allow: number_of_children-- only if not 0

From SF user negan07

process.c

@@ -123,7 +123,8 @@ process_handle_child_termination(int signal)
 			else
 				break;
 		}
-		number_of_children--;
+		if (number_of_children)
+			number_of_children--;
 		remove_process_info(pid);
 	}
 }