net/filter: free old values in property setters

netfilter_set_position and netfilter_set_netdev_id overwrote their
respective fields with g_strdup without freeing the previous value,
leaking memory on each property write.

Fixes: fdccce4596 ("init/cleanup of netfilter object")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

net/filter.c

@@ -130,6 +130,7 @@ static void netfilter_set_netdev_id(Object *obj, const char *str, Error **errp)
 {
     NetFilterState *nf = NETFILTER(obj);
 
+    g_free(nf->netdev_id);
     nf->netdev_id = g_strdup(str);
 }
 
@@ -182,6 +183,7 @@ static void netfilter_set_position(Object *obj, const char *str, Error **errp)
 {
     NetFilterState *nf = NETFILTER(obj);
 
+    g_free(nf->position);
     nf->position = g_strdup(str);
 }