In addtion to 1023, the GICC_IAR register may return 1022 as a special value.
Simply check for a valid interrupt vector for the dispatching.
Check the GICC_IAR again after the dispatch to quickly process a next interrupt
without having to go through the interrupt prologue and epiloge.
The existing code is functional but inccorrect and blindly modifies the
other masking bits. It is important to preserve those other bits since
they control masking of important system events.
