Fix cave map generator producing passages along the map border

Maps are 0-indexed even in Lua (so that 1 ends up as the lowest passable coordinate), so subtract 1 here

Closes #5407

data/lua/cave_map_generator.lua

@@ -122,7 +122,7 @@ function callbacks.generate_map(params)
 			local width = math.max(v.data.width or 1, 1)
 			local jagged = v.data.jagged or 0
 			local calc = function(x, y)
-				if x == 0 or x == params.map_width or y == 0 or y == params.map_height then
+				if x == 0 or x == params.map_width - 1 or y == 0 or y == params.map_height - 1 then
 					-- Map borders are impassable
 					return math.huge
 				end