Fix send buffer misalignment issue #4922

Job	Run time
Basic Checks	22m 16s
Measure coverage	29m 22s
miri	2m 58s
Additional Builds and Concurrency Tests	15m 2s
Release builds and tests	52m 49s
sanitize (address)	19m 29s
Slow tests	14m 5s
sanitize (address, multi-threading)	22m 3s
sanitize (leak)	9m 49s
sanitize (leak, multi-threading)	8m 49s
	3h 16m 42s

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