lxc: Don't crash on NULL ifname_guest_actual

Reported and patch provided by Bastian Blank at

    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=769600

src/lxc/lxc_container.c

@@ -472,7 +472,7 @@ lxcContainerGetNetDef(virDomainDefPtr vmDef, const char *devName)
 
     for (i = 0; i < vmDef->nnets; i++) {
         netDef = vmDef->nets[i];
-        if (STREQ(netDef->ifname_guest_actual, devName))
+        if (STREQ_NULLABLE(netDef->ifname_guest_actual, devName))
             return netDef;
     }