log: Plug memory leak on multiple -dfilter

-dfilter overwrites any previous filter.  The overwritten filter is
leaked.  Leaks since the beginning (commit 3514552, v2.6.0).  Free it
properly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1466011636-6112-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
