samples: net: dtls_client: Fix mem leak in error path

We need to unref the net_buf in error path in RX and TX.

Jira: ZEP-1169

Change-Id: Icb6d43cb6b7411a5135ea09c6ae96742566fafc4
Signed-off-by: Jukka Rissanen <[email protected]>
(cherry picked from commit 22ddc4419c6d7823cd530a5d5fcd1d508add4a50)

Author: jukkar

samples/net/mbedtls_dtlsclient/src/udp.c

@@ -89,6 +89,7 @@ int udp_tx(void *context, const unsigned char *buf, size_t size)
 	rc = net_nbuf_append(send_buf, size, (uint8_t *) buf, K_FOREVER);
 	if (!rc) {
 		printk("cannot write buf\n");
+		net_nbuf_unref(send_buf);
 		return -EIO;
 	}
 
@@ -121,6 +122,8 @@ int udp_rx(void *context, unsigned char *buf, size_t size)
 
 	read_bytes = net_nbuf_appdatalen(ctx->rx_nbuf);
 	if (read_bytes > size) {
+		net_nbuf_unref(ctx->rx_nbuf);
+		ctx->rx_nbuf = NULL;
 		return -ENOMEM;
 	}