opal_environ.c: ensure "value" is a valid string for the setenv() case

This was CID 1269764.

opal/util/opal_environ.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      Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2006-2015 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2007-2013 Los Alamos National Security, LLC.  All rights
  *                         reserved.
  * Copyright (c) 2014      Intel, Inc. All rights reserved.
@@ -101,6 +101,7 @@ int opal_setenv(const char *name, const char *value, bool overwrite,
     /* Make the new value */
 
     if (NULL == value) {
+        value = "";
         asprintf(&newvalue, "%s=", name);
     } else {
         asprintf(&newvalue, "%s=%s", name, value);