eliminate compiler warning #418

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	30m 0s
ci (a6osx, macos-13)	14m 46s
ci (ta6osx, macos-13)	18m 55s
ci (arm64osx, macos-14)	8m 52s
ci (tarm64osx, macos-14)	11m 42s
ci (i3le, ubuntu-22.04)	12m 42s
ci (ti3le, ubuntu-22.04)	16m 11s
ci (a6le, ubuntu-22.04)	8m 59s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	12m 3s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 14s
ci (i3nt, windows-2022, vs)	11m 40s
ci (ti3nt, windows-2022, vs)	15m 7s
ci (a6nt, windows-2022, vs)	12m 34s
ci (ta6nt, windows-2022, vs)	15m 16s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 6s
ci (ta6le, ubuntu-22.04, test-more, more)	58m 26s
	4h 28m 33s

Provide feedback