Fix memory leak when there are no network interfaces or an error occurs.

src/net_ifs.c

@@ -149,12 +149,13 @@ get_net_ifs(char **addrinfo)
 	}
     }
     if (num_interfaces == 0)
-	debug_return_int(0);
+	goto done;
     ailen = num_interfaces * 2 * INET6_ADDRSTRLEN;
     if ((cp = malloc(ailen)) == NULL) {
 	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO,
 	    "unable to allocate memory");
-	debug_return_int(-1);
+	num_interfaces = -1;
+	goto done;
     }
     *addrinfo = cp;