Passing non u8 list to u8-list->bytevector should raise an error

[Takashi Kato]

This should raise an error

(u8-list->bytevector '(a))

But returns ().

[Takashi Kato]

Raising &assertion instead of returning SG_NIL on Sg_ListToBytevector. (Fixes #115)

→ <<cset 560185f79fe9>>