conf: Avoid NULL dereference in virDomainNetPortForwardFree

conf: Avoid NULL dereference in virDomainNetPortForwardFree

In our current code the function is not called with NULL argument, but
we should follow our common practice and make it safe anyway.

Reported by coverity:

    /src/conf/domain_conf.c: 2635 in virDomainNetPortForwardFree()
    2629     {
    2630         size_t i;
    2631
    2632         if (pf)
    2633             g_free(pf->dev);
    2634
    >>>     CID 404359:  Null pointer dereferences  (FORWARD_NULL)
    >>>     Dereferencing null pointer "pf".
    2635         for (i = 0; i < pf->nRanges; i++)
    2636             g_free(pf->ranges[i]);
    2637
    2638         g_free(pf->ranges);
    2639         g_free(pf);
    2640     }

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>