CVE-2026-31778: ALSA: caiaq: fix stack out-of-bounds read in init_card

In the Linux kernel, the following vulnerability has been resolved:

ALSA: caiaq: fix stack out-of-bounds read in initcard

The loop creates a whitespace-stripped copy of the card shortname where len < sizeof(card->id) is used for the bounds check. Since sizeof(card->id) is 16 and the local id buffer is also 16 bytes, writing 16 non-space characters fills the entire buffer, overwriting the terminating nullbyte.

When this non-null-terminated string is later passed to sndcardsetid() -> copyvalididstring(), the function scans forward with while (nid && ...) and reads past the end of the stack buffer, reading the contents of the stack.

A USB device with a product name containing many non-ASCII, non-space characters (e.g. multibyte UTF-8) will reliably trigger this as follows:

BUG: KASAN: stack-out-of-bounds in copyvalididstring sound/core/init.c:696 [inline] BUG: KASAN: stack-out-of-bounds in sndcardsetidnolock+0x698/0x74c sound/core/init.c:718

The off-by-one has been present since commit bafeee5b1f8d ("ALSA: sndusbcaiaq: give better shortname") from June 2009 (v2.6.31-rc1), which first introduced this whitespace-stripping loop. The original code never accounted for the null terminator when bounding the copy.

Fix this by changing the loop bound to sizeof(card->id) - 1, ensuring at least one byte remains as the null terminator.