vt_core: don't draw the splash if a panic occurred

The shutdown splash draws over all the useful info if ddb(4) is
disabled. Don't draw the splash screen if we're rebooting because of a
panic.

Approved by: re (cperciva)
MFC after: 3 days

(cherry picked from commit bc83f414583ccaf337ef3b7c019df7727d6a3aae)
(cherry picked from commit 3aabec5c8c6a74a4e50640b399a825d9f7adca9a)