fix port-bol? return type (#751) #58

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	31m 24s
ci (a6osx, macos-12)	18m 47s
ci (ta6osx, macos-12)	23m 2s
ci (i3le, ubuntu-22.04)	18m 8s
ci (ti3le, ubuntu-22.04)	23m 49s
ci (a6le, ubuntu-22.04)	10m 13s
ci (ta6le, ubuntu-22.04)	20m 3s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	17m 37s
ci (i3nt, windows-2022, vs)	18m 42s
ci (ti3nt, windows-2022, vs)	15m 6s
ci (a6nt, windows-2022, vs)	23m 0s
ci (ta6nt, windows-2022, vs)	15m 55s
ci (ta6nt, windows-2022, gcc)	24m 30s
	4h 20m 16s

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