lib/zoap: Fix incorrect values for payload size

When retrieving the payload length of the payload, there was a mistake,
what was already in the buffer (buf->len) was being considered twice,
net_buf_tailroom() already takes buf->len into account, resulting in
incorrect values.

Change-Id: I3b78d55abca4a448cab5e035b9d66352a3c59110
Signed-off-by: Vinicius Costa Gomes <[email protected]>

Author: vcgomes

lib/iot/zoap/zoap.c

@@ -776,7 +776,7 @@ uint8_t *zoap_packet_get_payload(struct zoap_packet *pkt, uint16_t *len)
 	}
 
 	if (len) {
-		*len = net_buf_tailroom(frag) - frag->len;
+		*len = net_buf_tailroom(frag);
 	}
 
 	return pkt->start;