Added fix in viewnavigator.js that fixes back button issue on FireFox

I noticed that when using FireFox on the TV Listings example, that after you click the back button on at least 2 pages, it would crash with the error "viewDescriptor.animation = 'popEffect'". The error was caused by FireFox calling the script twice and popping the history object again setting it to null.

Fix was a if statement to check if the length of the array is not 1 before it pops.