cpu plugin: Fix potential buffer overflow.

cpu plugin: Fix potential buffer overflow.

```
In function 'cpu_commit_without_aggregation',
    inlined from 'cpu_commit' at src/cpu.c:563:5,
    inlined from 'cpu_read' at src/cpu.c:925:3:
src/cpu.c:534:50: note: directive argument in the range [0, 18446744073709551614]
  534 |       snprintf(cpu_num_str, sizeof(cpu_num_str), "%zu", cpu_num);
      |                                                  ^~~~~
src/cpu.c:534:7: note: 'snprintf' output between 2 and 21 bytes into a destination of size 16
  534 |       snprintf(cpu_num_str, sizeof(cpu_num_str), "%zu", cpu_num);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```