qemu: Remove possible NULL deref in debug output

Check for !dev->info.alias was done after a VIR_DEBUG() statement
that already tried to print - just flip sequence

src/qemu/qemu_hotplug.c

@@ -1887,14 +1887,14 @@ int qemuDomainChangeNetLinkState(virQEMUDriverPtr driver,
     int ret = -1;
     qemuDomainObjPrivatePtr priv = vm->privateData;
 
-    VIR_DEBUG("dev: %s, state: %d", dev->info.alias, linkstate);
-
     if (!dev->info.alias) {
         virReportError(VIR_ERR_OPERATION_FAILED, "%s",
                        _("can't change link state: device alias not found"));
         return -1;
     }
 
+    VIR_DEBUG("dev: %s, state: %d", dev->info.alias, linkstate);
+
     qemuDomainObjEnterMonitor(driver, vm);
 
     ret = qemuMonitorSetLink(priv->mon, dev->info.alias, linkstate);