vbox: network: make sure driver is not NULL in virRegisterNetworkDriver

libvirtd will report below error if it does not make sure driver was not NULL
in virRegisterNetworkDriver

$ libvirtd
2014-10-24 09:24:36.443+0000: 28876: info : libvirt version: 1.2.10
2014-10-24 09:24:36.443+0000: 28876: error : virRegisterNetworkDriver:549 : driver in virRegisterNetworkDriver must not be NULL
2014-10-24 09:24:36.443+0000: 28876: error : virDriverLoadModule:99 : Failed module registration vboxNetworkRegister
Signed-off-by: NShanzhi Yu <shyu@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

src/vbox/vbox_driver.c

@@ -152,7 +152,7 @@ int vboxNetworkRegister(void)
     if (VBoxCGlueInit(&uVersion) == 0)
         networkDriver = vboxGetNetworkDriver(uVersion);
 
-    if (virRegisterNetworkDriver(networkDriver) < 0)
+    if (networkDriver && virRegisterNetworkDriver(networkDriver) < 0)
         return -1;
     return 0;
 }