[Security] Crafted format input reaches fatal xvasprintf abort

Hello GNU m4 Maintainer,

I would like to report a potential security issue in m4 1.4.21.

The issue is in the format builtin path. User-controlled width and 
precision values are forwarded into xasprintf in src/format.c:

```c
str = xasprintf (fstart, width, prec, ARG_INT (argc, argv));
```

That eventually reaches lib/xvasprintf.c, where a failure path prints an 
error and aborts:

```c
fprintf (stderr, "vasprintf failed! format=\"%s\", errno=%s\n",
          format, errname);
abort ();
```

Passing INT_MIN as a width can trigger an EOVERFLOW path and terminate 
the process.

Affected area:
- src/format.c:246-270
- src/format.c:365-367
- lib/xvasprintf.c:97-127

If useful, I can provide a minimal PoC and reproduction output.

If you believe this is a valid security issue, please let me know your 
preferred remediation or coordinated disclosure process. If needed, I 
can also assist with CVE coordination after triage.

-- 
Best regards,
Zhihan Zheng
Beijing Institute of Technology