Fix ninja scroll freakout

# Problem
Many rapid sequential swipes will eventually cause peek mode to go haywire and display no pages and prevent all interaction from working.  The problem is especially apparent on slower devices, like an iPad 3.

# Solution
Add guards to prevent the requested index from getting outside the rendered pages.



CR: @timmccall-wf @lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @todbachman-wf