Zero-value retrier will break your stuff.

[flowchartsman]

It's been awhile since I wrote this module and, looking at it again while making it an actual module, I note a glaring problem: it allows you to create a zero-value Retrier, whereupon it will panic:

package main

import (
	"errors"
	"github.com/flowchartsman/retry"
)

func main() {
	r := retry.Retrier{}
	r.Run(func() error {
		return errors.New("nope")
	})
}

Results in

panic: invalid argument to Int63n

goroutine 1 [running]:
math/rand.(*Rand).Int63n(0xc000074090, 0x0, 0x0)
        /Users/awalker/opt/go/src/math/rand/rand.go:111 +0x11d
github.com/flowchartsman/retry.jitterDuration(0x0, 0x0)
        /Users/awalker/go/pkg/mod/github.com/flowchartsman/retry@v1.1.0/retry.go:126 +0x87
github.com/flowchartsman/retry.getnextBackoff(0x1, 0x0, 0x0, 0xc0000100f0)
        /Users/awalker/go/pkg/mod/github.com/flowchartsman/retry@v1.1.0/retry.go:112 +0x65
github.com/flowchartsman/retry.(*Retrier).RunContext(0xc000104f60, 0x10a36c0, 0xc0000140a0, 0xc000104f20, 0x16510f9d643f60d8, 0xc00004e748)
        /Users/awalker/go/pkg/mod/github.com/flowchartsman/retry@v1.1.0/retry.go:81 +0x7d
github.com/flowchartsman/retry.(*Retrier).Run(0xc00004e760, 0x108f770, 0x0, 0x0)
        /Users/awalker/go/pkg/mod/github.com/flowchartsman/retry@v1.1.0/retry.go:45 +0x6b
main.main()
        /Users/awalker/go/src/scratch/retrybug/main.go:11 +0x48

Because, as we have seen before values <=0 passed to rand.Int63 will fail, and a zero-value Retrier is, unsurprisingly, filled with zeros.

Looking back on it, I can see what I was going for: I was attempting to force the user to create an object that could be messed with after instantiation, thus making it safe to use concurrently as generator at runtime. A good idea, but allowing a panic condition is a poor oversight that's easy to hit, since I just found myself attempting to do so, and others doubtless will as well. I think the simplest fix is to add the same default checks that are in place in the NewRetrier() method in the `RunContext()