Dgraph crashes while trying to load data. #550
Description
I was able to reproduce the issue twice where probably l.water is nil in posting/list.go. This was while trying to load the 21million RDF's with the schema.
goroutine 4185 [running]:
panic(0xdf77a0, 0xc4200141f0)
/usr/local/go/src/runtime/panic.go:500 +0x1a1
github.com/dgraph-io/dgraph/posting.(*List).addMutation(0xc46442a9a0, 0x1492760, 0xc42325c570, 0xc467a14cc0, 0x14ee840, 0x5, 0x5beae0)
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/posting/list.go:335 +0x385
github.com/dgraph-io/dgraph/posting.(*List).AddMutationWithIndex(0xc46442a9a0, 0x1492760, 0xc42325c570, 0xc467a14cc0, 0x0, 0x0)
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/posting/index.go:172 +0x2f3
github.com/dgraph-io/dgraph/worker.runMutations(0x7f4a75979be0, 0xc42325c570, 0xc4678ec000, 0x693, 0x6a0, 0x0, 0x0)
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/worker/mutation.go:50 +0x24c
github.com/dgraph-io/dgraph/worker.(*node).processMutation(0xc420092120, 0x2, 0x7d1, 0x0, 0xc45490a000, 0x9dd3, 0x9dd3, 0x0, 0x0, 0x0, ...)
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/worker/draft.go:372 +0x13b
github.com/dgraph-io/dgraph/worker.(*node).process(0xc420092120, 0x2, 0x7d1, 0x0, 0xc45490a000, 0x9dd3, 0x9dd3, 0x0, 0x0, 0x0, ...)
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/worker/draft.go:405 +0x387
created by github.com/dgraph-io/dgraph/worker.(*node).processApplyCh
/home/ubuntu/go/src/github.com/dgraph-io/dgraph/worker/draft.go:444 +0x49b