Fix signedness comparison warning. #3902

Job	Run time
cmake-offline-googletest	4m 40s
cmake-offline-googletest-src	4m 30s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 0s
package-source	3m 35s
system-sexpp, sexpp shared libs on, rnp shared libs off	5m 58s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	5m 1s
system-sexpp, sexpp shared libs off, rnp shared libs on	9m 37s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	8m 2s
system-sexpp, sexpp shared libs off, rnp shared libs off	3m 13s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 25s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	10m 8s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	4m 12s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	12m 45s
package	4m 28s
debian-tests	2m 44s
	1h 26m 18s

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