bus: fix memleak on invalid message

Introduced in 6d586a13.
Reported by Felix Riemann in https://bugzilla.redhat.com/show_bug.cgi?id=1685286.

Reproducer:
for i in `seq 1 100`; do gdbus call --session -d org.freedesktop.systemd1 -m org.freedesktop.systemd1.Manager.StartUnit -o "/$(for x in `seq 0 28000`; do echo -n $x; done)" & done

(cherry picked from commit 3dec5201)