tiny refactor texpos methods

library/lua/gui.lua

@@ -915,8 +915,7 @@ local BASE_FRAME = {
 local function make_frame(name, double_line)
     local tp = function(offset)
         local texpos = dfhack.textures.getAsset('hack/data/art/border-'..name:lower()..'.png', offset)
-        if texpos == -1 then return nil end
-        return texpos
+        return texpos >= 0 and texpos or nil
     end
 
     local frame = copyall(BASE_FRAME)

plugins/lua/buildingplan/pens.lua

@@ -11,8 +11,7 @@ local to_pen = dfhack.pen.parse
 
 local tp = function(asset, offset)
     local texpos = dfhack.textures.getAsset(asset, offset)
-    if texpos == -1 then return nil end
-    return texpos
+    return texpos >= 0 and texpos or nil
 end
 
 function reload_pens()