Fix #5377: Saved game keeps crashing shortly after loading

Guard against null pointers in banner_paint() (#5379)

contributors.md

@@ -76,6 +76,7 @@ Includes all git commit authors. Aliases are GitHub user names.
 * Jonathan Haas (HaasJona)
 * Jake Breen (Haekb)
 * Marco Benzi Tobar (Lisergishnu)
+* Richard Jenkins (rwjuk)
 
 ## Toolchain
 * (Balletie) - macOS

src/openrct2/paint/map_element/banner.c

@@ -48,6 +48,10 @@ void banner_paint(uint8 direction, sint32 height, rct_map_element* map_element)
 
 	rct_scenery_entry* banner_scenery = get_banner_entry(gBanners[map_element->properties.banner.index].type);
 
+	if ((banner_scenery == (rct_scenery_entry *)-1) || (banner_scenery == NULL)) {
+		return;
+	}
+
 	direction += map_element->properties.banner.position;
 	direction &= 3;