Swiper calculating index incorrectly when children count changes

Which OS ?

Both Android and iOS.

Version

Which versions are you using:

• react-native-swiper v? 1.5.14
• react-native v0.?.? 0.55.4

Expected behaviour

Expect the component to calculate the correct index.

Actual behaviour

The component wrongly updates the index.

How to reproduce it>

To help us, please fork this component, modify one example in examples folder to reproduce your issue and include link here.

Steps to reproduce

1. Create a react-native-swiper with n children components and place some dot navigation markers.
2. Swipe to the last component of the swiper.
3. Update the swiper children with (n-1) components.
4. The component will render correctly.
5. Try to swipe back. The component will navigate accordingly, but the dots aren't updated correctly, because index is calculated wrongly.

Additional Notes

I've supposedly found the bug. I can make a PR if requested.

Use the reproduction steps in the issue with a React Native Swiper containing n children, then replace them with n-1 children and swipe back. The issue points to modifying an example in the examples folder; confirm that the rendered component and dot navigation both show the correct index after the child count changes.