Add assertions that kill() is never accidentally passed a non-positive

Add assertions that kill() is never accidentally passed a non-positive
pid.  This follows a bug in rsync where it would accidentally
kill(-1), removing all the user's processes.  I can't see any way this
would directly happen in Samba, but having the assertions seems
beneficial.

http://cvs.samba.org/cgi-bin/cvsweb/rsync/util.c.diff?r1=1.108&r2=1.109&f=h
(This used to be commit 098905bea29c7d5b886809d431294ddf2fc1e152)