[BUG] panic: runtime error: slice bounds out of range

[flucrezia]

Describe the bug
Panic when max-metrics-rendered is less than number of files in a single render call.
To reproduce:

• Set a (relative) small value of max-metrics-rendered in go-carbon.conf
• Set maxBatchSize = 0 in carbonapi config file
• Query thousands of metrics in a single render call (> than max-metrics-rendered)

Logs
**panic: runtime error: slice bounds out of range [:1000] with capacity 0

goroutine 7080414 [running]:
github.com/go-graphite/go-carbon/carbonserver.(*CarbonserverListener).prepareDataStream(0xc000208380, {0xf70c78, 0xc085090cf0}, 0x3, 0xc013e2e228?, 0x0?, 0xc008fcb740)
/usr/local/src/go-carbon/carbonserver/render.go:423 +0xf45
github.com/go-graphite/go-carbon/carbonserver.(CarbonserverListener).prepareDataProto.func1()
/usr/local/src/go-carbon/carbonserver/render.go:493 +0xb3
created by github.com/go-graphite/go-carbon/carbonserver.(CarbonserverListener).prepareDataProto
/usr/local/src/go-carbon/carbonserver/render.go:491 +0x4d5

Go-carbon Configuration:
max-metrics-rendered = 1000

Commenting the above line to use the default value avoids the runtime error.