fix a bug in the value of the FTEXT field of the gz header

[folkertdev]

this field is used as a hint, to indicate that the file is probably ASCII text. We checked for > 0, but should be checking for != 0 for consistency with zlib-ng.

See https://github.com/zlib-ng/zlib-ng/blob/2c98ece180e1b541f0311220e38937dd0a94d08e/deflate.c#L923.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ
fuzz-compress	40.87% <0.00%> (ø)
fuzz-decompress	29.57% <0.00%> (-1.46%)	⬇️
test-aarch64-apple-darwin	93.27% <100.00%> (ø)
test-x86_64-apple-darwin	91.72% <100.00%> (-0.01%)	⬇️
test-x86_64-unknown-linux-gnu	90.26% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
zlib-rs/src/c_api.rs	91.93% <100.00%> (ø)
zlib-rs/src/deflate.rs	96.95% <100.00%> (ø)

... and 1 file with indirect coverage changes

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