pstat linux: ensure read() returns >=0

This was CID 71182.

opal/mca/pstat/linux/pstat_linux_module.c

@@ -9,7 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
- * Copyright (c) 2006-2007 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2006-2015 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2013      Los Alamos National Security, LLC.  All rights reserved.
  * Copyright (c) 2013      Intel, Inc.  All rights reserved.
  *
@@ -327,6 +327,9 @@ static int query(pid_t pid,
         memset(data, 0, sizeof(data));
         len = read(fd, data, sizeof(data)-1);
         close(fd);
+        if (len < 0) {
+            goto diskstats;
+        }
     
         /* remove newline at end */
         data[len] = '\0';