Crash in appendDummyValues

[tzdybal]

Following query: {n(func:ge(_uid_, 0)) {count()}}
crash the server.

Stack trace:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xc02b60]

goroutine 19040363 [running]:
github.com/dgraph-io/dgraph/query.(*SubGraph).appendDummyValues(0xc4fadea280)
	/home/tomek/devel/go/src/github.com/dgraph-io/dgraph/query/query.go:1610 +0xa0
github.com/dgraph-io/dgraph/query.ProcessGraph(0x13d0340, 0xc42c434b40, 0xc4fadea280, 0x0, 0xc42c434ba0)
	/home/tomek/devel/go/src/github.com/dgraph-io/dgraph/query/query.go:1624 +0x2c4f
created by github.com/dgraph-io/dgraph/query.(*QueryRequest).ProcessQuery
	/home/tomek/devel/go/src/github.com/dgraph-io/dgraph/query/query.go:2320 +0x59f