panic when rollback to 0 in plasma

Failure in CI

panic: runtime error: index out of range

goroutine 438247 [running]:
panic(0x55960d0a66c0, 0xc420018130)
/home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:500 +0x1a1 fp=0xc42b697cb8 sp=0xc42b697c28
runtime.panicindex()
/home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:27 +0x6d fp=0xc42b697ce8 sp=0xc42b697cb8
github.com/couchbase/indexing/secondary/indexer.(*plasmaSlice).handleCommandsWorker(0xc43fb4ad80, 0x0)
goproj/src/github.com/couchbase/indexing/secondary/indexer/plasma_slice.go:477 +0x7a8 fp=0xc42b697f60 sp=0xc42b697ce8
runtime.goexit()
/home/buildbot/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/asm_amd64.s:2086 +0x1 fp=0xc42b697f68 sp=0xc42b697f60
created by github.com/couchbase/indexing/secondary/indexer.(*plasmaSlice).initWriters
goproj/src/github.com/couchbase/indexing/secondary/indexer/plasma_slice.go:2154 +0x3b3