make-der-integer doesn't accept negative value

[Takashi Kato]

This must be valid

(make-der-integer -1)

But got an &assertion.

And this must return #vu8(2 2 0 255)

(encode (make-der-integer #xFF))

But got #vu8(2 1 255)

[Takashi Kato]

Using sinteger->bytevector to convert integer to der-integer. (Fixes #210)

→ <<cset 7e9c4e4c55e1>>