Added null check for items being moved to scrollView with constraints to...

... prevent crash

I have an app that primarily uses auto layout, but for the contentView of the cell that's using BMXSwipableCell I made the decision to do my layout manually after toying with auto layout for a bit. I was getting crashes in `bmx_moveContentViewSubViews` because firstItem and secondItem were sometimes nil (this didn't happen until I transitioned from auto layout to manual). Adding this check seemed to fix the problem.