Replace Unsafe C String Function

The function `append` in `sniffer.c` uses the unsafe `vsprintf`, which can lead to buffer overflows if a large or malformed packet is processed.

**Task:**

* [ ] Replace `vsprintf` with the safer `vsnprintf` alternative, making sure to use the buffer's available space as the limit.

**Suggested code change**

```c
va_list ap;
va_start(ap, fmt);
vsnprintf(*dst + used, *cap - used, fmt, ap);
va_end(ap);
```

**Why:**
Using `vsnprintf` helps prevent buffer overflows and potential crashes or vulnerabilities.

**Reference:**
See [[CWE-242: Use of Inherently Dangerous Function](https://cwe.mitre.org/data/definitions/242.html)](https://cwe.mitre.org/data/definitions/242.html)