core: don't pass uninitialzed PIDs to pid_is_unwaited()

core: don't pass uninitialzed PIDs to pid_is_unwaited()

Since 5fd9b2c5467b0a42ccdabc7eb8e516d512609a8e passing a pid of 0 to
pid_is_unwaited() and pid_is_live() is considered as a request on the
current process, similar how the other calls in process-util.c handle a
PID of 0. This broke service.c, which passes a 0 PID and expects it to
be considered an unwaited process.

This fix make sure we can boot again.