Respect the unpack boundaries.

As Gilles suggested on #2535 the opal_unpack_general_function was
unpacking based on the requested count and not on the amount of packed
data provided.
Fixes #2535.

Signed-off-by: George Bosilca <bosilca@icl.utk.edu>

opal/datatype/opal_datatype_unpack.c

@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2006 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2014 The University of Tennessee and The University
+ * Copyright (c) 2004-2017 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2006 High Performance Computing Center Stuttgart,
@@ -502,6 +502,7 @@ opal_unpack_general_function( opal_convertor_t* pConvertor,
                     conv_ptr = pConvertor->pBaseBuf + pStack->disp;
                     pos_desc++;  /* advance to the next data */
                     UPDATE_INTERNAL_COUNTERS( description, pos_desc, pElem, count_desc );
+                    if( 0 == iov_len_local ) goto complete_loop;  /* escape if we're done */
                     continue;
                 }
                 conv_ptr += rc * description[pos_desc].elem.extent;