network: bridge: Avoid freeing uninitialized pointer on cleanup path

The cleanup path in networkBuildDhcpDaemonCommandLine could cause a
crash by freeing uninitialized pointer.

src/network/bridge_driver.c

@@ -1252,7 +1252,7 @@ networkBuildDhcpDaemonCommandLine(virNetworkObjPtr network,
     int ret = -1;
     char *configfile = NULL;
     char *configstr = NULL;
-    char *leaseshelper_path;
+    char *leaseshelper_path = NULL;
 
     network->dnsmasqPid = -1;