Reading bytevector from standard input port always return one element on Windows

[Takashi Kato]

This should return a bytevector with 5 elements:

(get-bytevector-n (standard-input-port) 5)

But got one element bytevector.

[Takashi Kato]

Removing ASSERT(size==1) (Fixes #157)

→ <<cset 4b8d8a6e10f2>>