samples: net: dtls_client: Fix Coverity warning

This fix is basically a no-op as the rx_buf pointer cannot be null
in practice, but in order to avoid Coverity complaining about
it add some null pointer checks to the UDP handling code.

Coverity-CID: 170124
Jira: ZEP-2235

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