Fix crash when switch the data source

[gshe]

crash (100%) steps:

1. set the ViewPagerController’s data source, total count is 10;
2. select the active page as 9;
3. i want to change the data source to another data source, total count maybe is 5. So i call ViewPagerController reloadData.
4. ViewPagerController crash, out of range exception

Root Cause is self.activeContentIndex is still 9, NSArray cash when call [self.contents replaceObjectAtIndex:index withObject:[NSNull null]]

[pplant]

Tried it on my fork and works like a charm! should be merged as soon as possible