handle invalid maxItems
handle invalid maxItems

for #1114

mtraynham/dc.js,aaronhoffman/dc.js,aaronhoffman/dc.js,TheDataShed/dc.js,mtraynham/dc.js,dc-js/dc.js,dc-js/dc.js,dc-js/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,dc-js/dc.js,aaronhoffman/dc.js,TheDataShed/dc.js,mtraynham/dc.js,aaronhoffman/dc.js,mtraynham/dc.js,TheDataShed/dc.js,TheDataShed/dc.js,TheDataShed/dc.js,dc-js/dc.js