fix bug in inflate_fast when refilling

[folkertdev]

fix #213

the logic assumed a certain number of bits would be available; this was false in general.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zlib-rs/src/inflate.rs	90.90%	1 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/inflate/bitreader.rs	90.62% <ø> (-0.99%)	⬇️
zlib-rs/src/inflate.rs	95.06% <90.90%> (-0.04%)	⬇️

... and 2 files with indirect coverage changes