flate: Remove unneeded bounds checks (effectively one less)

[klauspost]

(no measurable difference)

[orijbot]

Visit https://dashboard.github.orijtech.com?back=0&pr=439&remote=false&repo=klauspost%2Fcompress to see benchmark details.

[odeke-em]

@klauspost very interesting that despite your mention of no noticeable differences, on Linux the benchmarks show many improvements from that bounds removal per https://dashboard.github.orijtech.com/benchmark/f6cd51a3d9144c73954783b91ad4492d

[klauspost]

@odeke-em Half of these are unrelated, one is benchmarking an stdlib function and shows +6%

So to be honest I don't really trust these numbers.

Gunzip is completely unrelated.
Transport is benchmarking stdlib, nothing in this package.
DecodeRandom is decompression and unrelated.
2 x EncodeTwainConstant is unrelated.

So 5 of 9 is in unrelated code.

My own numbers of the differences above (n=1, though)

BenchmarkEncodeDigitsSpeed1e6-32        6565961       6561624       -0.07%
BenchmarkEncodeDigitsCompress1e6-32     30598651      30685776      +0.28%
BenchmarkEncodeDigitsSL1e4-32           51189         50338         -1.66%
BenchmarkEncodeTwainSpeed1e6-32         6500970       6524704       +0.37%

[klauspost]

My own numbers are way below the noise of my own system.