rpcrt4: Never allocate memory for the discriminant in union_arm_unmarshall as the memory is part of the union and has already been allocated.

rpcrt4: Check to make sure there is enough data in the buffer during unmarshalling, so that the code doesn't try to read beyound the end of the buffer.

rpcrt4: Even though FC_ENUM16 is an unsigned type, the highest allowable value that can be marshalled is SHRT_MAX, not USHRT_MAX.

CoFreeUnusedLibraries doesn't do anything and COM_FlushMessageQueue 
won't actually allow any RPCs to be processed as the apartment has 
already been destroyed.

This is needed to return the correct value when one of the handles
being waited on is set to signaled state by the APC.