lxc: Fix segfault when lxc.network does not start with 'type'

To configure network settings using config file, legacy LXC settings
require starting them with 'lxc.network.type' entry. If someone
accidentally starts with 'lxc.network.name', libvirt will crash with
segfault. This patch checks if this case is happening.

Sample invalid settings:
lxc.network.link = eth0
lxc.network.type = phys
lxc.network.name = eth1
lxc.network.ipv4 = 192.168.122.2/24
lxc.network.ipv4.gateway = 192.168.122.1

Now, libvirt only see error without segmentation fault.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

src/lxc/lxc_native.c

@@ -717,7 +717,11 @@ lxcNetworkGetParseDataByIndexLegacy(lxcNetworkParseDataArray *networks,
     }
 
     /* Return last element added like a stack. */
-    return networks->parseData[ndata - 1];
+    if (ndata > 0)
+        return networks->parseData[ndata - 1];
+
+    /* Not able to retrive an element */
+    return NULL;
 }