progresswidget(fix): ignore nil self.last

spec/unit/widget_progresswidget_spec.lua

@@ -0,0 +1,18 @@
+describe("ProgressWidget widget", function()
+    local ProgressWidget, Screen
+    setup(function()
+        require("commonrequire")
+        ProgressWidget = require("ui/widget/progresswidget")
+        Screen = require("device").screen
+    end)
+
+    it("should not crash with nil self.last #ok", function()
+        local progress = ProgressWidget:new{
+            width = 100,
+            height = 50,
+            percentage = 5/100,
+            ticks = {1},
+        }
+        progress:paintTo(Screen.bb, 0, 0)
+    end)
+end)