getvar: fix broken 'fastboot flash' when partition has slots

fastboot getvar current-slot > check slot is a or b, and my device show the slot on b fastboot set_active a > change the slot from b to a 8

Xiaomi Mi A2$ fastboot getvar current-slot current-slot: a finished total time: mf@mf-OptiPlex-3040:~PulpitXiaomi Mi A2$ fastboot

fastboot change slot fastboot getvar battery-status; fastboot getvar battery-voltage current partitions and flashing the backed-up images saved on your computer

fastboot getvar currentslot fastboot commands :- https How to Check and Change Current Active Slot on Android