qemu_driver.c: don't unlink(NULL) on OOM error path

* src/qemu/qemu_driver.c (qemudDomainMigratePrepareTunnel): Upon an
out of memory error, we would end up with unixfile==NULL and attempt
to unlink(NULL).  Skip the unlink when it's NULL.

src/qemu/qemu_driver.c

@@ -6903,7 +6903,8 @@ endjob:
 
 cleanup:
     virDomainDefFree(def);
-    unlink(unixfile);
+    if (unixfile)
+        unlink(unixfile);
     VIR_FREE(unixfile);
     if (vm)
         virDomainObjUnlock(vm);