Optimise _krb5_n_fold() a bit.

Optimise _krb5_n_fold() a bit.

All in lib/krb5/n-fold.c:

       1.  eliminate malloc/free from rr13() because it is always a
	   buffer of the same size called in a tight loop.

       2.  eliminate memcpy(3) from rr13() by bouncing back and forth
	   between two buffers buf1, buf2 instead of performing the
	   calculation into a tmp buffer and memcpy(3)ing the result
	   back into buf.

       3.  eliminate code cases from rr13() that I can visually determine
	   will never occur but I'm guessing that the compiler can't, i.e.

	       i.   now that we're no longer using malloc(3), rr13()
		    cannot fail, so make it void and avoid the if in
		    the calling routine checking its error code.  In
		    case you ask, yes, this made the tests run a little
		    faster,

	       ii.  rr13() has code for being passed a number of bits
		    not divisble by 8 but _krb5_n_fold() only passes
		    an int * 8.	 So, we eliminate this conditional and
		    the associated code.

       4.  we make rr13() take 2 destination buffers and copy the results
	   into both of them, we use this to eliminate another memcpy(3)
	   from the calling routine.  This appears to make it a bit faster
	   as well.