unit: fix undefined behaviour in prbuf test

The test start to fail in CI on osx_debug (x86_64) workflow

```
[033]  	*** test_buffer_foreach_copy_number ***
[033] -ok 13 - prbuf(size=256, payload=16, iterations=16) has been validated
[033] -ok 14 - prbuf(size=256, payload=16, iterations=32) has been validated
[033] -ok 15 - prbuf(size=256, payload=16, iterations=64) has been validated
[033] +ok 13 - prbuf(size=256, payload=4294967312, iterations=16) has been validated
[033] +ok 14 - prbuf(size=256, payload=4294967312, iterations=32) has been validated
[033] +ok 15 - prbuf(size=256, payload=4294967312, iterations=64) has been validated
[033]  	*** test_buffer_foreach_copy_number: done ***
```

NO_CHANGELOG=test fix
NO_DOC=test fix

(cherry picked from commit 4a868563)