cmdline:burn: always return true if burnt

cmdline:burn: always return true if burnt

Before we have been trying to cram three cases into a boolean return
value:

 * cmdline had secrets, we burnt them       ->  true
 * cmdline had no secrets, all good         ->  false
 * cmdline has NULL string, WTF! emergency! ->  false

This return value is only used by Python which wants to know whether to
go to the trouble of replacing the command line. If samba_cmdline_burn()
returns false, no action is taken.

If samba_cmdline_burn() burns a password and then hits a NULL, it would
be better not to do nothing. It would be better to crash. And that is
what Python will end up doing, by some talloc returning NULL triggering
a MemoryError.

What about the case like {"--foo", NULL, "-Ua%b"} where the secret comes
after the NULL? That will still be ignored by Python, as it is by all C
tools, but we are hoping that can't happen anyway.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15674

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>