Fix the saturated addition panic inside IPA #4851

Job	Run time
Basic Checks	15m 6s
Additional Builds and Concurrency Tests	11m 52s
miri	2m 55s
Measure coverage	16m 45s
Release builds and tests	10m 15s
sanitize (address)	13m 4s
Slow tests	6m 4s
sanitize (address, multi-threading)	13m 59s
sanitize (leak)	7m 59s
sanitize (leak, multi-threading)	5m 54s
	1h 43m 53s

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