bug: rofi calc does not `add to history` in combi mode

[zspher]

When selecting add to history in combi mode, rofi calc does not add entry to history

to reproduce

1. rofi -plugin-path "build/src" -combi-modes calc,drun -show combi -no-config
2. type 12+5
3. select add to history and press enter

additional info

• other entries work fine

[zspher]

speculation

add to history calls append_last_result_to_history() which uses pd->last_result.
The issue may be caused by pd->last_result never being set as in combi mode _preprocess_input (which sets pd->last_result ) is never called.