common.sh: fix unbound variable issue breaking boot on devices
This was seen on FP4 for example on last night's devel r408 build when lxc-android-config was started:
mount-android-partitions[4323]: Guessing /android/vendor
mount-android-partitions[4337]: /usr/lib/lxc-android-config/common.sh: line 32: ab_slot_suffix: unbound variable
systemd[1]: mount-android-partitions.service: Main process exited, code=exited, status=1/FAILURE
Fixes !114 (merged).