Summary:
SectionList accesses items outside of the array bounds.
This was discovered when using mobx, which warns you: `[mobx.array] Attempt to read an array index (${index}) that is out of bounds`. This is because `section.data[itemIndex + 1]` goes beyond array length.
This PR adds an array length check and simplifies the code a bit to avoid repetitive `this.props.`
Pull Request resolved: https://github.com/facebook/react-native/pull/23710
Differential Revision: D14298557
Pulled By: cpojer
fbshipit-source-id: fee3422ad5b053d91a097c5842f46e78a149c3d5
54f91d3ca9