Testapp sometimes crashes when navigating in the Hebrew sample book

[qnga]

(in R2EpubActivity.kt)
R2WebView.progression is negative, so positions[positionIndex] raises an out of bounds exception and the app crashes.

[mickael-menu]

There might be something to fix in the JavaScript calculating the progression with RTL books.

But we also need to guard against crashes if the positionIndex is invalid, with positions.indices.contains(positionIndex).