samples: net: zperf: Check UDP pkt before accessing it

Check that we do not try to access fragment when UDP packet is
received if pkt is NULL. In practice this should not happen for
UDP but do the checks in proper order anyway.

Coverity-CID: 179252
Fixes #5057

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>