dtls: Fix debug log

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_msg.c

diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index abb5a56..86c23d7 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -3618,7 +3618,7 @@
                               (
                                   "datagram of length %u too small to hold DTLS record header of length %u",
                                   (unsigned) len,
-                                  (unsigned) (rec_hdr_len_len + rec_hdr_len_len)));
+                                  (unsigned) (rec_hdr_len_offset + rec_hdr_len_len)));
         return MBEDTLS_ERR_SSL_INVALID_RECORD;
     }