[Libvir] Remove useless "if (foo)" before "free (foo)".

[Jim Meyering]

Jim Meyering <jim at meyering.net> wrote:
> Likewise, given if (foo != NULL) free (foo); remove the useless "if" test.
> Do the same for sexpr_free, since it has similar semantics.
...
>  25 files changed, 110 insertions(+), 207 deletions(-)

BTW, that's a big patch, and if you're inclined to review it, that'd
be great, because I made the changes manually.  However, I've just
realized I should be able to check most of it by automating the
transformation and then doing a whitespace-ignoring diff.