core: reset bus error before reuse

From a report in https://bugzilla.redhat.com/show_bug.cgi?id=1861463:
usb-gadget.target: Failed to load configuration: No such file or directory
usb-gadget.target: Failed to load configuration: No such file or directory
usb-gadget.target: Trying to enqueue job usb-gadget.target/start/fail
usb-gadget.target: Failed to load configuration: No such file or directory
Assertion '!bus_error_is_dirty(e)' failed at src/libsystemd/sd-bus/bus-error.c:239, function bus_error_setfv(). Ignoring.
sys-devices-platform-soc-2100000.bus-2184000.usb-ci_hdrc.0-udc-ci_hdrc.0.device: Failed to enqueue SYSTEMD_WANTS= job, ignoring: Unit usb-gadget.target not found.

I *think* this is the place where the reuse occurs: we call
bus_unit_validate_load_state(unit, e) twice in a row.