CLEANUP: opentracing: remove the last two occurrences of strncat()

CLEANUP: opentracing: remove the last two occurrences of strncat()

In flt_ot_sample_to_str() there were two occurrences of strncat() which
are used to copy N chars from the source and append a zero. For the sake
of definitely getting rid of this nasty function let's replace them by
memcpy() instead. It's worth noting that the length test there appeared
to be incorrect as it didn't make provisions for the trailing zero,
unless the size argument doesn't take it into account (seems unlikely).
Nothing was changed regarding this. If the code was good, it still is,
otherwise if it was bad it still is. At least this is more obvious now
than when using a function that needs n+1 chars to work.