All items in SYSTEM_PROCESSOR array may not be initialized

All items in SYSTEM_PROCESSOR array may not be initialized

The SYSTEM_PROCESSOR array is allocated to fit the maximum number of supported
CPUs, but elements are only initialized when the SystemProcessorChangeCallback
callback is called with KeProcessorAddCompleteNotify.
Check if the SYSTEM_PROCESSOR structure is initialized before accessing any
other members, and fail SystemProcessorVcpuId with STATUS_NOT_SUPPORTED for any
uninitialized CPUs

Signed-off-by: Owen Smith <owen.smith@citrix.com>