Set the number of elements 0 in virNetwork*Clear

Decrementing it when it was already 0 causes an invalid free
in virNetworkDefUpdateDNSHost if virNetworkDNSHostDefParseXML
fails and virNetworkDNSHostDefClear gets called twice.

virNetworkForwardDefClear left the number untouched even if it
freed all the elements.
(cherry picked from commit c4e23388)

src/conf/network_conf.c

@@ -134,8 +134,8 @@ virNetworkIpDefClear(virNetworkIpDefPtr def)
     VIR_FREE(def->family);
     VIR_FREE(def->ranges);
 
-    while (def->nhosts--)
-        virNetworkDHCPHostDefClear(&def->hosts[def->nhosts]);
+    while (def->nhosts)
+        virNetworkDHCPHostDefClear(&def->hosts[--def->nhosts]);
 
     VIR_FREE(def->hosts);
     VIR_FREE(def->tftproot);
@@ -152,8 +152,8 @@ virNetworkDNSTxtDefClear(virNetworkDNSTxtDefPtr def)
 static void
 virNetworkDNSHostDefClear(virNetworkDNSHostDefPtr def)
 {
-    while (def->nnames--)
-        VIR_FREE(def->names[def->nnames]);
+    while (def->nnames)
+        VIR_FREE(def->names[--def->nnames]);
     VIR_FREE(def->names);
 }
 
@@ -170,18 +170,18 @@ static void
 virNetworkDNSDefClear(virNetworkDNSDefPtr def)
 {
     if (def->txts) {
-        while (def->ntxts--)
-            virNetworkDNSTxtDefClear(&def->txts[def->ntxts]);
+        while (def->ntxts)
+            virNetworkDNSTxtDefClear(&def->txts[--def->ntxts]);
         VIR_FREE(def->txts);
     }
     if (def->hosts) {
-        while (def->nhosts--)
-            virNetworkDNSHostDefClear(&def->hosts[def->nhosts]);
+        while (def->nhosts)
+            virNetworkDNSHostDefClear(&def->hosts[--def->nhosts]);
         VIR_FREE(def->hosts);
     }
     if (def->srvs) {
-        while (def->nsrvs--)
-            virNetworkDNSSrvDefClear(&def->srvs[def->nsrvs]);
+        while (def->nsrvs)
+            virNetworkDNSSrvDefClear(&def->srvs[--def->nsrvs]);
         VIR_FREE(def->srvs);
     }
 }
@@ -200,6 +200,7 @@ virNetworkForwardDefClear(virNetworkForwardDefPtr def)
         virNetworkForwardIfDefClear(&def->ifs[ii]);
     }
     VIR_FREE(def->ifs);
+    def->nifs = def->npfs = 0;
 }
 
 void