common: fix pointer-before-start UB in drop_incomplete_utf8

[dxbjavid]

drop_incomplete_utf8() computes strchr(c, 0) - 1, which is undefined
when c is empty. The empty case is reached from
avahi_alternative_service_name(" #N") and avahi_alternative_host_name("-N")
via avahi_strndup(s, 0); fuzz-domain exercises both entry points.

[evverx]

As mentioned in another PR one-off fixes with no tests aren't considered especially in cases like this where tests are straightforward. I'll keep it open as a bug report though. Generally it would probably be better to open issues first and mention how issues are discovered along with all the details like the tooling, *san/Valgrind backtraces, the output of static analyzers and their names or anything like that.

I'll also point to https://github.com/avahi/avahi/blob/master/.github/CONTRIBUTING.md again

Our golden rule is that a contribution should be worth more to the project than the time it takes to review it.