Fix for issue 1207 - Can't revisit the same page after visitin it, hi…

…tting the back button and clicking its link

- This was a regression introduced by 20dce2c. Added check for undefined newActiveIndex so we can tell the difference between the URL not being found, and a zero index.

js/jquery.mobile.navigation.js

@@ -305,7 +305,7 @@
 			});
 
 			// save new page index
-			urlHistory.activeIndex = newActiveIndex ? newActiveIndex : urlHistory.activeIndex;
+			urlHistory.activeIndex = newActiveIndex !== undefined ? newActiveIndex : urlHistory.activeIndex;
 
 			//if it's a back, use reverse animation
 			if( back ){