memchecker.h: fix memchecker no-data case

Thanks to @clintonstimpson for reporting the issue.

Fixes open-mpi/ompi#100

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

ompi/include/ompi/memchecker.h

@@ -10,6 +10,7 @@
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2014      Intel, Inc. All rights reserved.
  *
+ * Copyright (c) 2016 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -100,6 +101,10 @@ static inline int memchecker_call (int (*f)(void *, size_t), const void * addr,
         return OMPI_SUCCESS;
     }
 
+    if ((0 == count) || (0 == datatype->super.size)) {
+        return OMPI_SUCCESS;
+    }
+
     if( datatype->super.size == (size_t) (datatype->super.true_ub - datatype->super.true_lb) ) {
         /*  We have a contiguous type. */
         f( (void*)addr , datatype->super.size * count );