Avoid deleting NULL veth device name

If veth device allocation has a fatal error, the veths
array may contain NULL device names. Avoid calling the
virNetDevVethDelete function on such names.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/lxc/lxc_process.c

@@ -1290,7 +1290,7 @@ cleanup:
         rc = -1;
     }
     for (i = 0; i < nveths; i++) {
-        if (rc != 0)
+        if (rc != 0 && veths[i])
             ignore_value(virNetDevVethDelete(veths[i]));
         VIR_FREE(veths[i]);
     }