selftest: Fix selftest where pid is used uninitialized.

selftest: Fix selftest where pid is used uninitialized.

On my system this gets evaluated to 0 so in the end we detect samba to
be running cause $childpid is set to 0.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10793

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Sep  4 17:09:17 CEST 2014 on sn-devel-104

(cherry picked from commit 6d2f56dbaf84203b351f33179cc3feaf557e0683)
Signed-off-by: Andreas Schneider <asn@samba.org>
Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Thu Sep 11 22:02:42 CEST 2014 on sn-devel-104