samples: net: dtls_client: Fix memory leak in RX path

If we have not yet handled the previous RX buf, then we need
to drop the latest received one, otherwise the earlier net_buf
is leaked.

Jira: ZEP-1169

Change-Id: I1b69e07e8b3a3b87c76d923c847dc8316c128e76
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
(cherry picked from commit 9527d4cdcbbf199d7d596ed18f4edb435cca4130)