source4/smbd/pidfile: don't panic if pid file is corrupt.

source4/smbd/pidfile: don't panic if pid file is corrupt.

In particular, on a virtual machine after a forced reboot, it
contained "Ille" instead of a valid PID.  Given it was the right
length, I'm assuming it was filesystem corruption.

process_exists_by_pid() then panics, when given a pid < 1.

Reported-by: lostogre on #samba-technical
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Autobuild-User(master): Rusty Russell <rusty@rustcorp.com.au>
Autobuild-Date(master): Thu Jun 28 05:19:24 CEST 2012 on sn-devel-104