[Security] Integer overflow in src/freeze.c frozen-state reload path

Hello GNU m4 Maintainer,

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

The issue is in frozen-state reload handling in src/freeze.c. The 
affected logic is:

```c
if (number[(i)] + 1 > allocated[(i)])
   {
     allocated[(i)] = number[(i)] + 1;
     string[(i)] = xcharalloc ((size_t) allocated[(i)]);
   }
```

The parsed length is kept in int. A crafted frozen file can supply a 
large value such as 2147483647, causing number[i] + 1 to overflow and 
leading to abnormal allocation behavior. On my test build this produces 
a deterministic denial of service during m4 -R processing.

Affected area:
- src/freeze.c:205-217
- src/freeze.c:245-259
- src/freeze.c:323-325

If useful, I can provide a minimal PoC and sanitizer 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