Fix: Status bar hides text with minimal bottom margin (#5135)

With very narrow bottom margin text is sometimes hidden by status bar.

frontend/apps/reader/modules/readerfooter.lua

@@ -295,7 +295,11 @@ function ReaderFooter:resetLayout()
 end
 
 function ReaderFooter:getHeight()
-    return self.footer_content:getSize().h
+    if self.footer_content then
+        return self.footer_content:getSize().h
+    else
+        return 0
+    end
 end
 
 function ReaderFooter:disableFooter()