prevent allocation size computation overflow

prevent allocation size computation overflow

On systems where size_t is unsigned int, computing the size
of the resp_times array can overflow.  Add a check to prevent
this.

On a 64-bit x86_64 Ubuntu 22.04.5 LTS system, the overflow check
results in a compiler warning:

```
fping.c: In function ‘add_addr’:
fping.c:3444:20: warning: comparison is always false due to limited range of data type [-Wtype-limits]
 3444 |         if (trials > (SIZE_MAX / sizeof(int64_t)))
      |                    ^
```

Thus limit the check to systems with a size_t equal to (or less
than) unsigned int.