fix selection of prerender pages in pdf viewer

services/web/public/coffee/ide/pdfng/directives/pdfViewer.coffee

@@ -265,13 +265,13 @@ define [
 					lastVisiblePage = visiblePages[len-1].pageNum
 					lastVisiblePageIdx = lastVisiblePage - 1
 					# first page after
-					if lastVisiblePageIdx + 1 < len
+					if lastVisiblePageIdx + 1 < scope.pages.length
 						extra.push scope.pages[lastVisiblePageIdx + 1]
 					# page before
 					if firstVisiblePageIdx > 0
 						extra.push scope.pages[firstVisiblePageIdx - 1]
 					# second page after
-					if lastVisiblePageIdx + 2 < len
+					if lastVisiblePageIdx + 2 < scope.pages.length
 						extra.push scope.pages[lastVisiblePageIdx + 2]
 					return visiblePages.concat extra