availableScreenRect only for valid screens

* don't ask corona for screengeometry for the -1 screen
* in availableScreenRect don't ask corona for screens > than corona()->numScreens()

This solves some of the "Requesting unexisting screen" spam messages,
while doesn't change behavior of old plasmashell.

The second half of fixing the unexisting screen messages has to be done
in ShellCorona and is adressed in
plasma-workspace!2277
where makes sure to minimize the emission of screenGeometryChanged/AvailableScreenGeometryChanged etc