Catch error in cleanup() function.

server.c

@@ -53,7 +53,11 @@ static void cleanup(void)
                 free(ships[i]);
         }
 
-        close(sockfd);
+        if (close(sockfd) < 0) {
+		perror(pname);
+		return 1;
+	}
+
         free(ai);
 }