net: lwm2m: Do not use snprintk() to get debugging token

It's highly unlikely that snprintk() will return a negative value, but
that's a possibility that will make the `pos` variable be set to a
value outside the boundaries of the statically allocated `buf` array.

Also clamp writes to ensure that the statically allocated buffer won't
be overwritten with a large token length.

Fixes #7070.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>