Fix AttrOperation::nextMultiparm to update the modified "exists" variable
Fix AttrOperation::nextMultiparm to update the modified "exists" variable

sonictk/HoudiniEngineForMaya,sideeffects/HoudiniEngineForMaya,sonictk/HoudiniEngineForMaya,sideeffects/HoudiniEngineForMaya,sideeffects/HoudiniEngineForMaya,pokyfudywise/HoudiniEngineForMaya,pokyfudywise/HoudiniEngineForMaya,sonictk/HoudiniEngineForMaya,pokyfudywise/HoudiniEngineForMaya