Fixing a crash on rollups

[i3149]

Fixing a crash on rollups:

anic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x10 pc=0x103f21a48]

goroutine 98 [running]:
github.com/kentik/ktranslate/pkg/rollup.(*StatsRollup).exportSum(0x14000676000, 0x14000ec0000, 0x0?, 0x102c1c334?, 0x140004939e0?, 0x0?, 0x0?)
	/Users/pye/src/ktranslate/pkg/rollup/stats.go:305 +0x428
created by github.com/kentik/ktranslate/pkg/rollup.(*StatsRollup).sumKvs in goroutine 30
	/Users/pye/src/ktranslate/pkg/rollup/stats.go:271 +0x168
zsh: exit 2     ./bin/ktranslate -nf.source=auto -rollup_interval 30 -filters  -rollups 

I guess no one ever used this code brach /shrug