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 <[email protected]>
(cherry picked from commit 9527d4cdcbbf199d7d596ed18f4edb435cca4130)

Author: jukkar

samples/net/mbedtls_dtlsclient/src/udp.c

@@ -52,8 +52,22 @@ static void udp_received(struct net_context *context,
 	ARG_UNUSED(context);
 	ARG_UNUSED(status);
 
+	if (ctx->rx_nbuf) {
+		k_sem_give(&ctx->rx_sem);
+		k_yield();
+
+		if (ctx->rx_nbuf) {
+			printk("Packet %p is still being handled, "
+			       "dropping %p\n", ctx->rx_nbuf, buf);
+
+			net_nbuf_unref(buf);
+			return;
+		}
+	}
+
 	ctx->rx_nbuf = buf;
 	k_sem_give(&ctx->rx_sem);
+	k_yield();
 }
 
 int udp_tx(void *context, const unsigned char *buf, size_t size)