QV4String: properly detect overflow when trying to convert to an array index

A wrong overflow detection caused strings like "240000000000" to pass
the conversion, even though they would not fit into a uint when
converted into base-10. This mis-conversion to uint then caused
all sorts of side effects (broken comparisons, wrong listing of
properties, and so on).

So, properly fix the overflow detection by using our numeric private
functions.

Change-Id: Icbf67ac68cf5785d6c77b433c7a45aed5285a8c2
Task-number: QTBUG-56830
Reviewed-by: Lars Knoll <lars.knoll@qt.io>