Fix behavior of `vector-copy` in SRFI 43

[jpellegrini]

The procedure should augment the vector as necessary, even if there's no fill argument...

(define v (make-vector 5 -1))
v => #(-1 -1 -1 -1 -1)

(vector-copy v 0 8)

The above currently triggers an error, but SRFI 43 says it should augment the vector with an unspecified argument...

This patch fixes this:

(vector-copy v 0 8) => #(-1 -1 -1 -1 -1 #void #void #void)

[egallesio]

Thanks @jpellegrini. Merged.