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.

MFC after:	3 days

sys/dev/vt/vt_core.c

@@ -1684,6 +1684,9 @@ vtterm_splash(struct vt_device *vd)
 	uintptr_t image;
 	vt_axis_t top, left;
 
+	if (KERNEL_PANICKED())
+		return;
+
 	if ((vd->vd_flags & VDF_TEXTMODE) != 0 || (boothowto & RB_MUTE) == 0)
 		return;