QIdentityProxyModel: remove slow bounds-checking, for more performance

If we're called out of bounds, sourceModel()->index() will take care of
returning an invalid model index anyway. So calling rowCount+columnCount
every time index() is called can be avoided. These calls can be particularly
slow when sitting on top of a stack of proxymodels. And index() itself is called
very often, i.e. when a proxymodel on top of us is used by a delegate
which calls data() for many different roles.

Change-Id: I00dd5cf432c0591f41e6fa6fc86c845f29845cd1
(cherry picked from qtbase/3de0f442b5857915f26be6600bc8e54d1af08208)
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>