Fixed incorrect return code handling in ssl3_final_finish_mac.

Based on an original patch by Joel Sing (OpenBSD) who also originally
identified the issue.

(Imported from upstream's 728bd41a159ea16a60111e7c1120ec2a005507b3)

ssl/s3_enc.c

@@ -608,10 +608,18 @@ int ssl3_cert_verify_mac(SSL *s, int md_nid, unsigned char *p)
 int ssl3_final_finish_mac(SSL *s, 
 	     const char *sender, int len, unsigned char *p)
 	{
-	int ret;
+	int ret, sha1len;
 	ret=ssl3_handshake_mac(s,NID_md5,sender,len,p);
+	if(ret == 0)
+		return 0;
+
 	p+=ret;
-	ret+=ssl3_handshake_mac(s,NID_sha1,sender,len,p);
+
+	sha1len=ssl3_handshake_mac(s,NID_sha1,sender,len,p);
+	if(sha1len == 0)
+		return 0;
+
+	ret+=sha1len;
 	return(ret);
 	}