taskmgr: Remove redundant out-of-domain checking which is already handled by PerfDataGetProcessorUsage() and PerfDataGetProcessorSystemUsage().

The implementation pointer retrieved from interface ptr wasn't being used for
anything except retrieving same interface ptr back.

It's clear that someone left it in by accident; the TABLET_FindOpenContext
is its clear successor.

ntdll: Set the DataLength field of the information field in NtQueryValueKey to how much data should be returned, not what was actually returned.

Fix NdrFreeBuffer to use the fBufferValid flag to determine whether or 
not I_RpcFreeBuffer needs to be called.