index out of range [-2] errors on Zipfian generator

[filipecosta90]

I'm seeing deterministic errors around the Zipfian generation when I need to generate hundreds of millions of random numbers.
Here's a sample:

panic: runtime error: index out of range [-2]
goroutine 1387 [running]:
math/rand.(*rngSource).Uint64(...)
	/usr/local/go/src/math/rand/rng.go:249
math/rand.(*rngSource).Int63(0xc0000d45a0?)
	/usr/local/go/src/math/rand/rng.go:234 +0x92
math/rand.(*Rand).Int63(...)
	/usr/local/go/src/math/rand/rand.go:96
math/rand.(*Rand).Float64(...)
	/usr/local/go/src/math/rand/rand.go:207
github.com/pingcap/go-ycsb/pkg/generator.(*Zipfian).next(0xc49960f560, 0xc0000e1830, 0x1)
	/go/pkg/mod/github.com/pingcap/[email protected]/pkg/generator/zipfian.go:151 +0x4e
github.com/pingcap/go-ycsb/pkg/generator.(*Zipfian).Next(...)
	/go/pkg/mod/github.com/pingcap/[email protected]/pkg/generator/zipfian.go:169

In the next couple of days I'll isolate this and reply back but wanted to have this placeholder to keep track of this issue.