bug#18866: bytevector-copy creates srfi-4 vector with greater length

[tantalum]

with guile version 2.1.0.89-c5ea7 on an x86_64 GNU/Linux system
and the following code
  (use-modules (srfi srfi-4) (rnrs bytevectors))
  (define a (make-f32vector 2 0))
  (define b (bytevector-copy a))
  (write (list a b))

"b" turns out to be an f32vector with length 8, 4 times the length of "a".
i expected the result to have the same length as the argument

see also http://lists.gnu.org/archive/html/guile-devel/2014-10/msg00064.html