Only attempt removal of the rule allowing tftp if it was added

During virtual network startup, the iptables rule that allows tftp
traffic is only added if network->def->tftproot is non-empty, but when
the virtual network is destroyed, we had been unconditionally trying
to delete the rule. This was harmless, except that it created a bogus
error message.

This patch conditionalizes the delete command in the same manner that
the insert command is already conditionalized.

src/network/bridge_driver.c

@@ -990,7 +990,8 @@ networkRemoveIptablesRules(struct network_driver *driver,
     iptablesRemoveForwardAllowCross(driver->iptables, network->def->bridge);
     iptablesRemoveForwardRejectIn(driver->iptables, network->def->bridge);
     iptablesRemoveForwardRejectOut(driver->iptables, network->def->bridge);
-    iptablesRemoveUdpInput(driver->iptables, network->def->bridge, 69);
+    if (network->def->tftproot)
+        iptablesRemoveUdpInput(driver->iptables, network->def->bridge, 69);
     iptablesRemoveUdpInput(driver->iptables, network->def->bridge, 53);
     iptablesRemoveTcpInput(driver->iptables, network->def->bridge, 53);
     iptablesRemoveUdpInput(driver->iptables, network->def->bridge, 67);