Panic when name list is empty

[richardmarshall]

The file namer doesn't check that the list of names actually contains any entries.

echo -n "" | ./dist/dnsbench_linux_amd64_v1/dnsbench run
# requests errors min  [ p50  p95  p99  p999] max  qps
panic: runtime error: integer divide by zero

goroutine 18 [running]:
github.com/askmediagroup/dnsbench/pkg.FileNamer.func1()
        github.com/askmediagroup/dnsbench/pkg/namer.go:32 +0x6a
github.com/askmediagroup/dnsbench/pkg.(*RemoteResolver).Resolve(0xc0000bc120, 0xc0000aaea0)
        github.com/askmediagroup/dnsbench/pkg/resolver.go:52 +0x85
github.com/askmediagroup/dnsbench/pkg.(*Runner).run(0xc0000b0240, 0x0?, 0x0?, 0x0?, 0x0?)
        github.com/askmediagroup/dnsbench/pkg/runner.go:131 +0x85
created by github.com/askmediagroup/dnsbench/pkg.(*Runner).Start
        github.com/askmediagroup/dnsbench/pkg/runner.go:63 +0x7d