Added comment for the frag->reassembly == NULL case as per feedback from Emilia

Reviewed-by: Emilia Käsper <emilia@openssl.org>

(Imported from upstream's ea7cb53974)

Change-Id: Idd5ed233028c42d2b921deb424381aad88a0aa84
Reviewed-on: https://boringssl-review.googlesource.com/1433
Reviewed-by: Adam Langley <agl@google.com>

ssl/d1_both.c

@@ -645,14 +645,14 @@ dtls1_reassemble_fragment(SSL *s, struct hm_header_st* msg_hdr, int *ok)
 			}
 		}
 
-
 	/* If message is already reassembled, this must be a
-	 * retransmit and can be dropped.
-	 */
+	 * retransmit and can be dropped. In this case item != NULL and so frag
+	 * does not need to be freed. */
 	if (frag->reassembly == NULL)
 		{
 		unsigned char devnull [256];
 
+		assert(item != NULL);
 		while (frag_len)
 			{
 			i = s->method->ssl_read_bytes(s,SSL3_RT_HANDSHAKE,