kconfig: nconf: Ensure null termination where strncpy is used

strncpy() does not guarantee null-termination if the source string is
longer than the destination buffer.

Ensure the buffer is explicitly null-terminated to prevent potential
string overflows or undefined behavior.

Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Nicolas Schier <n.schier@avm.de>

scripts/kconfig/nconf.c

@@ -593,6 +593,8 @@ static void item_add_str(const char *fmt, ...)
 		tmp_str,
 		sizeof(k_menu_items[index].str));
 
+	k_menu_items[index].str[sizeof(k_menu_items[index].str) - 1] = '\0';
+
 	free_item(curses_menu_items[index]);
 	curses_menu_items[index] = new_item(
 			k_menu_items[index].str,

scripts/kconfig/nconf.gui.c

@@ -359,6 +359,7 @@ int dialog_inputbox(WINDOW *main_window,
 	x = (columns-win_cols)/2;
 
 	strncpy(result, init, *result_len);
+	result[*result_len - 1] = '\0';
 
 	/* create the windows */
 	win = newwin(win_lines, win_cols, y, x);