network: Fix NULL dereference during error recovery

This fixes: https://bugzilla.redhat.com/show_bug.cgi?id=696660

While starting a network, if brSetForwardDelay() fails, we go to err1
where we want to access macTapIfName variable which was just
VIR_FREE'd a few lines above. Instead, keep macTapIfName until we are
certain of success.

src/network/bridge_driver.c

@@ -1616,7 +1616,7 @@ networkStartNetworkDaemon(struct network_driver *driver,
     bool v4present = false, v6present = false;
     virErrorPtr save_err = NULL;
     virNetworkIpDefPtr ipdef;
-    char *macTapIfName;
+    char *macTapIfName = NULL;
 
     if (virNetworkObjIsActive(network)) {
         networkReportError(VIR_ERR_OPERATION_INVALID,
@@ -1657,7 +1657,6 @@ networkStartNetworkDaemon(struct network_driver *driver,
             VIR_FREE(macTapIfName);
             goto err0;
         }
-        VIR_FREE(macTapIfName);
     }
 
     /* Set bridge options */
@@ -1731,6 +1730,7 @@ networkStartNetworkDaemon(struct network_driver *driver,
         goto err5;
     }
 
+    VIR_FREE(macTapIfName);
     VIR_INFO(_("Starting up network '%s'"), network->def->name);
     network->active = 1;
 
@@ -1778,6 +1778,7 @@ networkStartNetworkDaemon(struct network_driver *driver,
                  macTapIfName, network->def->bridge,
                  virStrerror(err, ebuf, sizeof ebuf));
     }
+    VIR_FREE(macTapIfName);
 
  err0:
     if (!save_err)