Don't overwhelm the blockLength.

During the optimization process the datatype engine will merge
contiguous elements. When this happen adding two integers might result
in a "largher than int" value and the resulting datatype will be
incorrectly sized. Use size_t instead of int (this is correct because
the MPI standard mandates the block lengths to be non-negative numbers).

Thanks @albandil for finding the issue and providing a patch.

Fixes #11607.

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

Author: bosilca

ompi/datatype/ompi_datatype_create_struct.c

@@ -33,8 +33,8 @@ int32_t ompi_datatype_create_struct( int count, const int* pBlockLength, const p
 {
     ptrdiff_t disp = 0, endto, lastExtent, lastDisp;
     ompi_datatype_t *pdt, *lastType;
-    int lastBlock;
     int i, start_from;
+    size_t lastBlock;
 
     /* Find first non-zero length element */
     for( i = 0; (i < count) && (0 == pBlockLength[i]); i++ );

test/datatype/opal_ddt_lib.c

@@ -365,8 +365,7 @@ static int32_t opal_datatype_create_struct(int count, const int *pBlockLength,
                                            opal_datatype_t **newType)
 {
     int i;
-    ptrdiff_t disp = 0, endto, lastExtent, lastDisp;
-    int lastBlock;
+    ptrdiff_t disp = 0, endto, lastExtent, lastDisp, lastBlock;
     opal_datatype_t *pdt, *lastType;
 
     if (0 == count) {