Make 112_backtrace/bcheck_123 more robust

Make 112_backtrace/bcheck_123 more robust

the strcat checker first checks dest for overlap, then src.
If the padding byte between b[] and the pad[] arrays happens to be zero
the dest check would have succeeded and the src test failed.  If that
padding byte would be zero the dest check would trigger first.
As we can't influence the padding byte (only the b[] and pad[] arrays)
it was random if the dest or src checks triggered.

This makes it reliably trigger the dest check first.