Rp amd64 fixes

Some minor fixes to edge cases that were using amd64 emulator. Issue was that self.op is not initialized in EmulatorWorkspace but set in runFunction. There was a race where Amd64WorkspaceEmulator would check on the value of self.op before it was set in the EmulatorWorkspace base class. Initialized it to None, and added a check within getRegister to ensure self.op != None before continuing.