BatchSliceOfStrings(): panic if count <= 0

[Al2Klimov]

Otherwise, 0 would lead to infinite empty slices.
Negative numbers already crash the program.

[oxzi]

As I have tested and documented over at #32 (comment), both a count of zero as well as a negative count only do harm.

[yhabteab]

Currently it is only used here:

icinga-go-library/redis/client.go

Line 163 in 15703e6

batches := utils.BatchSliceOfStrings(ctx, fields, c.Options.HMGetCount)

and redis.Options.HMGetCount is not allowed to be zero or a negative number, so this should be fine.

icinga-go-library/redis/config.go

Lines 24 to 26 in 15703e6

	if o.HMGetCount < 1 {
	return errors.New("hmget_count must be at least 1")
	}