add a number of additional (debug) asserts to spot invalid states earlier

[folkertdev]

I'm not sure that we should merge this (we'd at least need to look at performance), but it could be useful for debugging

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zlib-rs/src/deflate.rs	71.42%	2 Missing ⚠️

Files with missing lines	Coverage Δ
zlib-rs/src/read_buf.rs	90.90% <100.00%> (+0.43%)	⬆️
zlib-rs/src/deflate.rs	96.87% <71.42%> (-0.06%)	⬇️