jcc: panic in CorrectMissingHeaders(), trying to copy a file that does not exist

[cjx10]

From build log:
/usr/local/bin/clang++-jcc -std=gnu++20 -DHAVE_CONFIG_H -I. -I./src -I./src -DKJ_HEADER_WARNINGS -DCAPNP_HEADER_WARNINGS -DCAPNP_INCLUDE_DIR='"/usr/local/include"' -O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize-address-use-after-scope -fsanitize=fuzzer-no-link -stdlib=libc++ -DKJ_HAS_ZLIB -DKJ_HAS_OPENSSL -DKJ_USE_FIBERS -MT src/capnp/compat/json-test.capnp.o -MD -MP -MF $depbase.Tpo -c -o src/capnp/compat/json-test.capnp.o src/capnp/compat/json-test.capnp.c++ &&\ mv -f $depbase.Tpo $depbase.Po

panic: open 5: no such file or directory

goroutine 1 [running]:
main.CopyFile({0xc000172023?, 0xa0?}, {0xc00015c0c0, 0x11})
	jcc.go:38 +0x74
main.GetHeaderCorrectedCmd({0xc000154008, 0x1f, 0x3f}, {0xc000172000?, 0x3f?})
	jcc.go:144 +0x1c6
main.CorrectMissingHeaders({0x505874, 0x7}, {0xc000154008, 0x1f, 0x3f})
	jcc.go:154 +0x59
main.main()
	jcc.go:401 +0x449