libxl: only close fds which successfully opened in libxl__spawn_local_dm

Coverity-ID: 1055565
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_dm.c

@@ -1259,8 +1259,8 @@ retry_transaction:
     rc = 0;
 
 out_close:
-    close(null);
-    close(logfile_w);
+    if (null != -1) close(null);
+    if (logfile_w != -1) close(logfile_w);
 out:
     if (rc)
         device_model_spawn_outcome(egc, dmss, rc);