nwfilter: remove pointless assignment

Changing a parameter passed by value has no effect.

Introduced by <commit 3f74b2eb>.

Signed-off-by: Ján Tomko <jtomko@redhat.com>

src/nwfilter/nwfilter_gentech_driver.c

@@ -360,7 +360,6 @@ virNWFilterRuleDefToRuleInst(virNWFilterDefPtr def,
                            inst->nrules,
                            ruleinst) < 0)
         goto cleanup;
-    inst = NULL;
 
     ret = 0;
  cleanup: