nwfilter: Avoid possible double free in virNWFilterInstReset()

virNWFilterInstReset() may be called multiple times, leading to a double g_free()
Replace plain g_free() with g_clear_pointer() to prevent this

Found by Linux Verification Center (linuxtesting.org) with Svace.

Reported-by: Dmitry Fedin <d.fedin@fobos-nt.ru>
Signed-off-by: Alexander Kuznetsov <kuznetsovam@altlinux.org>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/nwfilter/nwfilter_gentech_driver.c

@@ -203,12 +203,12 @@ virNWFilterInstReset(virNWFilterInst *inst)
 
     for (i = 0; i < inst->nfilters; i++)
         virNWFilterObjUnlock(inst->filters[i]);
-    g_free(inst->filters);
+    g_clear_pointer(inst->filters, g_free);
     inst->nfilters = 0;
 
     for (i = 0; i < inst->nrules; i++)
         virNWFilterRuleInstFree(inst->rules[i]);
-    g_free(inst->rules);
+    g_clear_pointer(inst->rules, g_free);
     inst->nrules = 0;
 }