From 4d8028831bbf4daf4dec1744775d13778ea3e5d9 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <coelckers@users.noreply.github.com>
Date: Wed, 29 Dec 2021 23:41:24 +0100
Subject: [PATCH] - fixed vertexscan crashing on bad walls.

---
 source/core/gamefuncs.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/source/core/gamefuncs.h b/source/core/gamefuncs.h
index 74fd7d8f6..f2235178a 100644
--- a/source/core/gamefuncs.h
+++ b/source/core/gamefuncs.h
@@ -138,7 +138,8 @@ void vertexscan(walltype* startwall, func mark)
 	while (true)
 	{
 		auto thelastwall = wal->lastWall();
-		if (!thelastwall->twoSided()) break;
+		// thelastwall can be null here if the map is bogus.
+		if (!thelastwall || !thelastwall->twoSided()) break;
 
 		wal = thelastwall->nextWall();
 		if (walbitmap.Check(wall.IndexOf(wal))) break;