`put-bytevector' should allow zero-sized writes

[Andreas Rottmann]

Ikarus doesn't allow zero-sized writes with `put-bytevector', which
should by legal, at least according to my reading of R6RS.

  Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision 1771, build 2009-05-08)
  Copyright (c) 2006-2009 Abdulaziz Ghuloum

  > (put-bytevector (standard-output-port) #vu8() 0)
  Unhandled exception
   Condition components:
     1. &assertion
     2. &who: put-bytevector
     3. &message: "index out of range"
     4. &irritants: (0)
  > (put-bytevector (standard-output-port) #vu8() 0 0)
  Unhandled exception
   Condition components:
     1. &assertion
     2. &who: put-bytevector
     3. &message: "index out of range"
     4. &irritants: (0)
  >

The attached patch should fix this.

[Abdulaziz Ghuloum (aghuloum)]

Applied patch in revision 1790.