externalterm: fix SIGNED_INT_TO_UNSIGNED warning #2017

Job	Run time
run-tests (ubuntu-20.04, -s prime_smp, 21)	2m 43s
run-tests (ubuntu-20.04, -s prime_smp, 22)	1m 59s
run-tests (ubuntu-20.04, -s prime_smp, 23)	1m 55s
run-tests (ubuntu-22.04, -s prime_smp, 24)	2m 38s
run-tests (ubuntu-22.04, -s prime_smp, 25)	2m 48s
run-tests (ubuntu-22.04, -s prime_smp, 26, erlang:26)	3m 14s
run-tests (macos-latest, 23, /usr/local/opt/erlang@23/bin:, -DOPENSSL_ROOT_DIR=/usr/local/opt/ope...	4m 4s
run-tests (macos-latest, 24, /usr/local/opt/erlang@24/bin:, -DOPENSSL_ROOT_DIR=/usr/local/opt/ope...	6m 7s
run-tests (macos-latest, 25, /usr/local/opt/erlang@25/bin:, -DOPENSSL_ROOT_DIR=/usr/local/opt/ope...	5m 8s
	30m 36s

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