sec/base: fix coverity issues

CID 1292483 Uninitialized pointer read (UNINIT)

Initialize the method and credential members of the opal_sec_cred_t to
avoid possible invalid read when calling cleanup_cred.

CID 1292484 Double free (USE_AFTER_FREE)

Set method and credential members to NULL after freeing in
cleanup_cred.

Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

opal/mca/sec/base/sec_base_stubs.c

@@ -1,5 +1,8 @@
+/* -*- Mode: C; c-basic-offset:4 ; indent-tabs-mode:nil -*- */
 /*
  * Copyright (c) 2014-2015 Intel, Inc. All rights reserved.
+ * Copyright (c) 2015      Los Alamos National Security, LLC. All rights
+ *                         reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -26,9 +29,11 @@ static void cleanup_cred(opal_sec_cred_t *cred)
     }
     if (NULL != cred->method) {
         free(cred->method);
+        cred->method = NULL;
     }
     if (NULL != cred->credential) {
         free(cred->credential);
+        cred->credential = NULL;
     }
 }
 
@@ -99,7 +104,7 @@ int opal_sec_base_validate(char *payload, size_t size, char **method)
     opal_sec_handle_t *hdl;
     opal_buffer_t buf;
     int cnt, rc;
-    opal_sec_cred_t cred;
+    opal_sec_cred_t cred = {.method = NULL, .credential = NULL};
     
     opal_output_verbose(5, opal_sec_base_framework.framework_output,
                         "opal_sec: Received credential of size %lu",