Fix test of first of 255 CBC padding bytes.

Thanks to Peter Gijsels for pointing out that if a CBC record has 255
bytes of padding, the first was not being checked.

(This is an import of change 80842bdb from BoringSSL.)
Reviewed-by: NEmilia Käsper <emilia@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1431)

ssl/record/ssl3_record.c

@@ -1149,9 +1149,9 @@ int tls1_cbc_remove_padding(const SSL *s,
      * maximum amount of padding possible. (Again, the length of the record
      * is public information so we can use it.)
      */
-    to_check = 255;             /* maximum amount of padding. */
-    if (to_check > rec->length - 1)
-        to_check = rec->length - 1;
+    to_check = 256;            /* maximum amount of padding, inc length byte. */
+    if (to_check > rec->length)
+        to_check = rec->length;
 
     for (i = 0; i < to_check; i++) {
         unsigned char mask = constant_time_ge_8(padding_length, i);