mpi pp: fix transmission chunk send

[JiakunYan]

Fix a bug in mpi parcelport due to the change of #6580.

Thanks to @m-diers for reporting the bug.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.60%	✅ 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (62d92ad)	237187	203187	85.67%
Head commit (9c014f0)	192960 (-44227)	164140 (-39047)	85.06% (-0.60%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6619)	1	0	0.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[m-diers]

Many thanks for the fix. It works for me as well.