Fix send buffer misalignment [No 2] #4975

Job	Run time
Measure coverage	26m 55s
miri	2m 57s
Additional Builds and Concurrency Tests	10m 45s
Basic Checks	20m 12s
Release builds and tests	13m 8s
sanitize (address)	19m 51s
Slow tests	13m 45s
sanitize (address, multi-threading)	21m 52s
sanitize (leak)	8m 22s
sanitize (leak, multi-threading)	10m 0s
	2h 27m 47s

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