vt: fix splash_cpu logos use of vd_drawrect

In the (extremely unlikely) case of vd->vd_height ==
vt_logo_sprite_height the vd_drawrect code would write outside of
frame-buffer memory.

MFC after: 1 week
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D34220

(cherry picked from commit 06296f77c5bc5ec1710a28b68b9410d192163087)