zstd: Skip random data faster

[klauspost]

If we did not get a single match, assume that the data cannot be entropy compressed.

It is very likely that the data is random, and in the rare case it is not, we can live with it.

Before:

BenchmarkRandomEncodeAllFastest-12           100          10475190 ns/op        1001.01 MB/s           0 B/op          0 allocs/op
BenchmarkRandomEncodeAllDefault-12           100          11075653 ns/op         946.74 MB/s           0 B/op          0 allocs/op
BenchmarkRandomEncoderFastest-12             200           8126950 ns/op        1290.25 MB/s         139 B/op          2 allocs/op
BenchmarkRandomEncoderDefault-12             200           8059989 ns/op        1300.96 MB/s          90 B/op          2 allocs/op

After:

BenchmarkRandomEncodeAllFastest-12           300           4425967 ns/op        2369.15 MB/s           0 B/op          0 allocs/op
BenchmarkRandomEncodeAllDefault-12           300           5108791 ns/op        2052.49 MB/s           0 B/op          0 allocs/op
BenchmarkRandomEncoderFastest-12             300           5085627 ns/op        2061.84 MB/s          89 B/op          2 allocs/op
BenchmarkRandomEncoderDefault-12             300           5947031 ns/op        1763.19 MB/s          65 B/op          2 allocs/op