Same fix as in dtls1_process_out_of_seq_message.

Applying same fix as in dtls1_process_out_of_seq_message. A truncated
DTLS fragment would cause *ok to be clear, but the return value would
still be the number of bytes read.

Problem identified by Emilia Käsper, based on previous issue/patch by Adam
Langley.

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

(Imported from upstream's 3d5dceac43)

Change-Id: Ibe30716266e2ee1489c98b922cf53edda096c23c
Reviewed-on: https://boringssl-review.googlesource.com/1437
Reviewed-by: Adam Langley <agl@google.com>

ssl/d1_both.c

@@ -670,7 +670,9 @@ dtls1_reassemble_fragment(SSL *s, struct hm_header_st* msg_hdr, int *ok)
 	/* read the body of the fragment (header has already been read */
 	i = s->method->ssl_read_bytes(s,SSL3_RT_HANDSHAKE,
 		frag->fragment + msg_hdr->frag_off,frag_len,0);
-	if (i<=0 || (unsigned long)i!=frag_len)
+	if ((unsigned long)i!=frag_len)
+		i=-1;
+	if (i<=0)
 		goto err;
 
 	RSMBLY_BITMASK_MARK(frag->reassembly, (long)msg_hdr->frag_off,