nwfilter: loop generated too many rules

The loop processing the trusted DHCP server generated one too
many rules and added one final rules that accepted responses
from all DHCP servers. Below patch fixes this.

src/nwfilter/nwfilter_ebiptables_driver.c

@@ -3374,10 +3374,10 @@ ebtablesApplyDHCPOnlyRules(const char *ifname,
 
         VIR_FREE(srcIPParam);
 
-        if (idx == num_dhcpsrvrs)
-            break;
-
         idx++;
+
+        if (idx >= num_dhcpsrvrs)
+            break;
     }
 
     virBufferAsprintf(&buf,