test/mbuf: fix forged mbuf in clone test

test/mbuf: fix forged mbuf in clone test

The test for cloning changed mbuf would generate an mbuf whose length
and segments count were invalid.
This would cause a crash if test was run with mbuf debugging enabled.

Fixes: 4ccd2bb3a9e2 ("app/test: enhance mbuf refcnt check")
Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Olivier Matz <olivier.matz@6wind.com>