tty: vt: replace BUG_ON() by WARN_ON_ONCE()

No need to panic in vc_uniscr_copy_line(), just warn. This should never
happen though, as vc_uniscr_check() is supposed to be called before
vc_uniscr_copy_line(). And the former checks vc->vc_uni_lines already.

In any case, use _ONCE as vc_uniscr_copy_line() is called repeatedly for
each line. So don't flood the logs, just in case.
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230112080136.4929-7-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -535,7 +535,8 @@ void vc_uniscr_copy_line(const struct vc_data *vc, void *dest, bool viewed,
 	int offset = row * vc->vc_size_row + col * 2;
 	unsigned long pos;
 
-	BUG_ON(!uni_lines);
+	if (WARN_ON_ONCE(!uni_lines))
+		return;
 
 	pos = (unsigned long)screenpos(vc, offset, viewed);
 	if (pos >= vc->vc_origin && pos < vc->vc_scr_end) {