zeroconf-publish: Don't assume any particular defer event ordering

Also, initialize userdata with zeros to avoid invalid pointers in
client_free().

This fixes a crash when client_free() is called before
create_client(). The whole issue could be avoided by using some other
mechanism than defer events for running the two functions, but I'll
do that change later (I have also other cleanups planned for
zeroconf-publish).

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=76184