vs_split() uses sp's visual map without checking it exists.

OK millert

usr.bin/vi/vi/vs_split.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: vs_split.c,v 1.18 2026/04/20 10:30:02 tb Exp $	*/
+/*	$OpenBSD: vs_split.c,v 1.19 2026/04/22 15:54:08 renaud Exp $	*/
 
 /*-
  * Copyright (c) 1993, 1994
@@ -43,6 +43,12 @@ vs_split(SCR *sp, SCR *new, int ccl)
 
 	gp = sp->gp;
 
+	/* Source screen must have a visual map to split. */
+	if (VIP(sp) == NULL || _HMAP(sp) == NULL) {
+		msgq(sp, M_ERR, "Not in visual mode: cannot split screen");
+		return (1);
+	}
+
 	/* Check to see if it's possible. */
 	/* XXX: The IS_ONELINE fix will change this, too. */
 	if (sp->rows < 4) {