Upon close reset the search_dirs.

This commit was SVN r32237.

opal/util/show_help.c

@@ -75,6 +75,7 @@ int opal_show_help_finalize(void)
     /* destruct the search list */
     if (NULL != search_dirs) {
         opal_argv_free(search_dirs);
+        search_dirs = NULL;
     };
     
     return OPAL_SUCCESS;