plain[tmsglen-1] needs sanity check

We want to perform a sanity check on the pad length plain[tmsglen-1], so we know it can not be more than the blocksize.

The range check would be against the message block size for the particular encryption algorithm used.  The kg_confounder_size() function returns this value.  If the value returned is less than the padding value given then we know that this is an error.